clutter/stage: Add API to paint to a custom target

Either onto a framebuffer, or into a CPU memory buffer. The latter will
use an former API and then copy the result to CPU memory. The former
allocates an offscreen framebuffer, sets up the relevant framebuffer
matrices and paints part of the stage defined by the passed rectangle.

This will be used by a RecordArea screen cast API. The former to paint
directly onto PipeWire handled dma-buf framebuffers, and the latter for
PipeWire handled shared memory buffers.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1207

clutter/clutter/clutter-mutter.h

@@ -48,6 +48,23 @@ void clutter_stage_capture_into (ClutterStage          *stage,
                                  cairo_rectangle_int_t *rect,
                                  uint8_t               *data);
 
+CLUTTER_EXPORT
+void clutter_stage_paint_to_framebuffer (ClutterStage                *stage,
+                                         CoglFramebuffer             *framebuffer,
+                                         const cairo_rectangle_int_t *rect,
+                                         float                        scale,
+                                         ClutterPaintFlag             paint_flags);
+
+CLUTTER_EXPORT
+gboolean clutter_stage_paint_to_buffer (ClutterStage                 *stage,
+                                        const cairo_rectangle_int_t  *rect,
+                                        float                         scale,
+                                        uint8_t                      *data,
+                                        int                           stride,
+                                        CoglPixelFormat               format,
+                                        ClutterPaintFlag              paint_flags,
+                                        GError                      **error);
+
 CLUTTER_EXPORT
 void clutter_stage_freeze_updates (ClutterStage *stage);
 

clutter/clutter/clutter-stage.c

@@ -4164,6 +4164,97 @@ clutter_stage_get_capture_final_size (ClutterStage          *stage,
   return TRUE;
 }
 
+void
+clutter_stage_paint_to_framebuffer (ClutterStage                *stage,
+                                    CoglFramebuffer             *framebuffer,
+                                    const cairo_rectangle_int_t *rect,
+                                    float                        scale,
+                                    ClutterPaintFlag             paint_flags)
+{
+  ClutterStagePrivate *priv = stage->priv;
+  ClutterPaintContext *paint_context;
+  cairo_region_t *redraw_clip;
+
+  redraw_clip = cairo_region_create_rectangle (rect);
+  paint_context =
+    clutter_paint_context_new_for_framebuffer (framebuffer,
+                                               redraw_clip,
+                                               paint_flags);
+  cairo_region_destroy (redraw_clip);
+
+  cogl_framebuffer_push_matrix (framebuffer);
+  cogl_framebuffer_set_projection_matrix (framebuffer, &priv->projection);
+  cogl_framebuffer_set_viewport (framebuffer,
+                                 -(rect->x * scale),
+                                 -(rect->y * scale),
+                                 priv->viewport[2] * scale,
+                                 priv->viewport[3] * scale);
+  clutter_actor_paint (CLUTTER_ACTOR (stage), paint_context);
+  cogl_framebuffer_pop_matrix (framebuffer);
+
+  clutter_paint_context_destroy (paint_context);
+}
+
+gboolean
+clutter_stage_paint_to_buffer (ClutterStage                 *stage,
+                               const cairo_rectangle_int_t  *rect,
+                               float                         scale,
+                               uint8_t                      *data,
+                               int                           stride,
+                               CoglPixelFormat               format,
+                               ClutterPaintFlag              paint_flags,
+                               GError                      **error)
+{
+  ClutterBackend *clutter_backend = clutter_get_default_backend ();
+  CoglContext *cogl_context =
+    clutter_backend_get_cogl_context (clutter_backend);
+  int texture_width, texture_height;
+  CoglTexture2D *texture;
+  CoglOffscreen *offscreen;
+  CoglFramebuffer *framebuffer;
+  CoglBitmap *bitmap;
+
+  texture_width = (int) ceilf (rect->width * scale);
+  texture_height = (int) ceilf (rect->height * scale);
+  texture = cogl_texture_2d_new_with_size (cogl_context,
+                                           texture_width,
+                                           texture_height);
+  if (!texture)
+    {
+      g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                   "Failed to create %dx%d texture",
+                   texture_width, texture_height);
+      return FALSE;
+    }
+
+  offscreen = cogl_offscreen_new_with_texture (COGL_TEXTURE (texture));
+  framebuffer = COGL_FRAMEBUFFER (offscreen);
+
+  cogl_object_unref (texture);
+
+  if (!cogl_framebuffer_allocate (framebuffer, error))
+    return FALSE;
+
+  clutter_stage_paint_to_framebuffer (stage, framebuffer,
+                                      rect, scale, paint_flags);
+
+  bitmap = cogl_bitmap_new_for_data (cogl_context,
+                                     texture_width, texture_height,
+                                     format,
+                                     stride,
+                                     data);
+
+  cogl_framebuffer_read_pixels_into_bitmap (framebuffer,
+                                            0, 0,
+                                            COGL_READ_PIXELS_COLOR_BUFFER,
+                                            bitmap);
+
+  cogl_object_unref (bitmap);
+  cogl_object_unref (framebuffer);
+
+  return TRUE;
+}
+
 static void
 capture_view_into (ClutterStage          *stage,
                    gboolean               paint,