clutter/stage-view: Ignore clipping rectangle for offscreen blit

In `clutter_stage_view_blit_offscreen()`, the given clipping rectangle
is in “view” coordinates whereas we intend to copy the whole actual
framebuffer, meaning that we cannot use the clipping rectangle.

Use the actual framebuffer size, starting at (0, 0) instead.

That fixes the issue with partial repainting with shadow framebuffer
when fractional scaling is enabled.

!820

clutter/clutter/clutter-stage-view.c

@@ -146,11 +146,14 @@ clutter_stage_view_blit_offscreen (ClutterStageView            *view,
   clutter_stage_view_get_offscreen_transformation_matrix (view, &matrix);
   if (cogl_matrix_is_identity (&matrix))
     {
+      int fb_width = cogl_framebuffer_get_width (priv->framebuffer);
+      int fb_height = cogl_framebuffer_get_height (priv->framebuffer);
+
       if (cogl_blit_framebuffer (priv->offscreen,
                                  priv->framebuffer,
-                                 rect->x, rect->y,
-                                 rect->x, rect->y,
-                                 rect->width, rect->height,
+                                 0, 0,
+                                 0, 0,
+                                 fb_width, fb_height,
                                  NULL))
         return;
     }