textview: use pixelcache rendered area to inform invalidation region.

textview: use pixelcache rendered area to inform invalidation region.

Use the pixelcache rendered area to inform what part of the cache should
be invalidated upon changes to the underlying textlayout.

By rendering the background to the pixelcache, we can avoid the need to
use RGBA content.

Also, we're using the pixel cache on the text windows bin_window (see
gtk_text_view_get_window) so we need to register the invalidation handler
on that, otherwise the region passed to the invalidate handler will get
clipped to the visible region.

https://bugzilla.gnome.org/show_bug.cgi?id=707244