cursor-renderer/native: Unpremultiply for color state transformation

Without doing this, a non-linear color state transformation could result
in premultiplied colour values larger than the alpha value, which
manifested with artifacts such as parts of the cursor shining brighter
than SDR white (with HDR enabled).

This was reported in the GNOME Shell Matrix room on August 8th 2024, and
I later hit it myself.

v2:
* Fix add_pipeline_snippet function formatting. (Sebastian Wick)

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4224>

src/backends/native/meta-cursor-renderer-native.c

@@ -89,6 +89,9 @@ struct _MetaCursorRendererNativePrivate
   gboolean input_disconnected;
   GMutex input_mutex;
   GCond input_cond;
+
+  CoglSnippet *premult_snippet;
+  CoglSnippet *unpremult_snippet;
 };
 typedef struct _MetaCursorRendererNativePrivate MetaCursorRendererNativePrivate;
 
@@ -183,6 +186,8 @@ meta_cursor_renderer_native_finalize (GObject *object)
   g_clear_signal_handler (&priv->texture_changed_handler_id,
                           priv->current_cursor);
   g_clear_object (&priv->current_cursor);
+  g_clear_object (&priv->premult_snippet);
+  g_clear_object (&priv->unpremult_snippet);
   g_clear_handle_id (&priv->animation_timeout_id, g_source_remove);
 
   G_OBJECT_CLASS (meta_cursor_renderer_native_parent_class)->finalize (object);
@@ -725,6 +730,40 @@ load_cursor_sprite_gbm_buffer_for_crtc (MetaCursorRendererNative *native,
   return TRUE;
 }
 
+static void
+add_pipeline_snippet (CoglPipeline  *pipeline,
+                      CoglSnippet  **snippet,
+                      const char    *snippet_source)
+{
+  if (!*snippet)
+    *snippet = cogl_snippet_new (COGL_SNIPPET_HOOK_FRAGMENT, "",
+                                 snippet_source);
+
+  cogl_pipeline_add_snippet (pipeline, *snippet);
+}
+
+static void
+add_pipeline_premultiply (MetaCursorRendererNative *cursor_renderer_native,
+                          CoglPipeline             *pipeline)
+{
+  MetaCursorRendererNativePrivate *priv =
+    meta_cursor_renderer_native_get_instance_private (cursor_renderer_native);
+
+  add_pipeline_snippet (pipeline, &priv->premult_snippet,
+                        "  cogl_color_out.rgb *= cogl_color_out.a;\n");
+}
+
+static void
+add_pipeline_unpremultiply (MetaCursorRendererNative *cursor_renderer_native,
+                            CoglPipeline             *pipeline)
+{
+  MetaCursorRendererNativePrivate *priv =
+    meta_cursor_renderer_native_get_instance_private (cursor_renderer_native);
+
+  add_pipeline_snippet (pipeline, &priv->unpremult_snippet,
+                        "  cogl_color_out.rgb /= cogl_color_out.a;\n");
+}
+
 static CoglTexture *
 scale_and_transform_cursor_sprite_cpu (MetaCursorRendererNative *cursor_renderer_native,
                                        ClutterColorState        *target_color_state,
@@ -772,11 +811,17 @@ scale_and_transform_cursor_sprite_cpu (MetaCursorRendererNative *cursor_renderer
   cogl_pipeline_set_layer_texture (pipeline, 0, src_texture);
   cogl_pipeline_set_layer_matrix (pipeline, 0, matrix);
 
+  if (cogl_texture_get_premultiplied (src_texture))
+    add_pipeline_unpremultiply (cursor_renderer_native, pipeline);
+
   color_state = meta_cursor_sprite_get_color_state (cursor_sprite);
   clutter_color_state_add_pipeline_transform (color_state,
                                               target_color_state,
                                               pipeline);
 
+  if (cogl_texture_get_premultiplied (dst_texture))
+    add_pipeline_premultiply (cursor_renderer_native, pipeline);
+
   cogl_framebuffer_clear4f (COGL_FRAMEBUFFER (offscreen),
                             COGL_BUFFER_BIT_COLOR,
                             0.0f, 0.0f, 0.0f, 0.0f);