Window Screencast Fixes

This MR fixes a few outstanding issues with window screencasting. The most important commit is:

window-actor: Clip before translate when blitting

cogl_framebuffer_push_rectangle_clip() acts on the current modelview
matrix. That means the result of clipping then translating will be
different of the result of translating then clipping.

What we want for window screencasting if the former, not the latter.
Move the translation code (and associated) to after clipping.

Fixes: #1097 (closed)