frame: Pass the frame area for get_mask()

Currently, `meta_frame_get_mask()` and `meta_ui_frame_get_mask()` will
return the frame mask applied to the current frame size, by querying the
frame themselves.

To be able to get the frame mask at an arbitrary size, change the API to
take a rectangle representing the size at which the frame mask should be
rendered.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/1009

src/compositor/meta-window-actor-x11.c

@@ -890,6 +890,9 @@ build_and_scan_frame_mask (MetaWindowActorX11    *actor_x11,
     {
       cairo_region_t *frame_paint_region, *scanned_region;
       cairo_rectangle_int_t rect = { 0, 0, tex_width, tex_height };
+      cairo_rectangle_int_t frame_rect;
+
+      meta_window_get_frame_rect (window, &frame_rect);
 
       /* Make sure we don't paint the frame over the client window. */
       frame_paint_region = cairo_region_create_rectangle (&rect);
@@ -899,7 +902,7 @@ build_and_scan_frame_mask (MetaWindowActorX11    *actor_x11,
       gdk_cairo_region (cr, frame_paint_region);
       cairo_clip (cr);
 
-      meta_frame_get_mask (window->frame, cr);
+      meta_frame_get_mask (window->frame, &frame_rect, cr);
 
       cairo_surface_flush (image);
       scanned_region = scan_visible_region (mask_data, stride, frame_paint_region);

src/core/frame.c

@@ -372,9 +372,10 @@ meta_frame_get_frame_bounds (MetaFrame *frame)
 
 void
 meta_frame_get_mask (MetaFrame             *frame,
+                     cairo_rectangle_int_t *frame_rect,
                      cairo_t               *cr)
 {
-  meta_ui_frame_get_mask (frame->ui_frame, cr);
+  meta_ui_frame_get_mask (frame->ui_frame, frame_rect, cr);
 }
 
 void

src/core/frame.h

@@ -73,6 +73,7 @@ void meta_frame_clear_cached_borders (MetaFrame *frame);
 cairo_region_t *meta_frame_get_frame_bounds (MetaFrame *frame);
 
 void meta_frame_get_mask (MetaFrame             *frame,
+                          cairo_rectangle_int_t *frame_rect,
                           cairo_t               *cr);
 
 void meta_frame_set_screen_cursor (MetaFrame	*frame,

src/ui/frames.c

@@ -1423,22 +1423,20 @@ get_visible_frame_border_region (MetaUIFrame *frame)
  * frame's alpha channel, as a mask to be applied to an opaque pixmap.
  *
  * @frame: This frame
- * @xwindow: The X window for the frame, which has the client window as a child
+ * @frame_rect: The frame rect
  * @cr: Used to draw the resulting mask
  */
 void
 meta_ui_frame_get_mask (MetaUIFrame           *frame,
+                        cairo_rectangle_int_t *frame_rect,
                         cairo_t               *cr)
 {
   MetaFrameBorders borders;
   MetaFrameFlags flags;
-  MetaRectangle frame_rect;
   cairo_surface_t *surface;
   double xscale, yscale;
   int scale;
 
-  meta_window_get_frame_rect (frame->meta_window, &frame_rect);
-
   flags = meta_frame_get_flags (frame->meta_window->frame);
 
   meta_style_info_set_flags (frame->style_info, flags);
@@ -1453,11 +1451,11 @@ meta_ui_frame_get_mask (MetaUIFrame *frame,
   gtk_render_background (frame->style_info->styles[META_STYLE_ELEMENT_FRAME], cr,
                          borders.invisible.left / scale,
                          borders.invisible.top / scale,
-                         frame_rect.width / scale, frame_rect.height / scale);
+                         frame_rect->width / scale, frame_rect->height / scale);
   gtk_render_background (frame->style_info->styles[META_STYLE_ELEMENT_TITLEBAR], cr,
                          borders.invisible.left / scale,
                          borders.invisible.top / scale,
-                         frame_rect.width / scale, borders.total.top / scale);
+                         frame_rect->width / scale, borders.total.top / scale);
 
   cairo_surface_set_device_scale (surface, xscale, yscale);
 }

src/ui/frames.h

@@ -136,6 +136,7 @@ void meta_ui_frame_get_borders (MetaUIFrame      *frame,
 cairo_region_t * meta_ui_frame_get_bounds (MetaUIFrame *frame);
 
 void meta_ui_frame_get_mask (MetaUIFrame           *frame,
+                             cairo_rectangle_int_t *frame_rect,
                              cairo_t               *cr);
 
 void meta_ui_frame_move_resize (MetaUIFrame *frame,