Fix the blur effect with fractional scaling

Commit message of the important commit:

blur-effect: Fix framebuffer sizes when stage-view scaling is used

When blurring only the actor (ACTOR mode), we don't want to apply any scale, it looks fine without using the resource scale and it also seems like clutter_actor_continue_paint in paint_actor_offscreen only draws an unscaled texture anyway (ie. if the resource scale is 2, only a quarter of the framebuffer is being drawn to).

In BACKGROUND mode though, we need to scale the framebuffer using the scale factor of the stage view (ie. the final scale factor for the monitor) because the content of the framebuffer we blit is scaled using that factor. Also, since the framebuffer we blit belongs to a stage view and only includes the contents of this view, we need to adjust the stage-coordinates of the actor to be relative to the stage-view.

To make sure we don't have to get the transformed actor size or position multiple times during one paint-run and don't have to carefully floor() or ceil() widths and positions, store the size of the actor (which is also the size of the framebuffer) and its position relative to the stage view inside a ClutterActorBox.

Fixes #2239 (closed)