gl-renderer: Rename buffer_damage variable

The buffer_damage variable stores accumulated damage from previous
frames. This is the area that, before considering our current repaint
request, we need to repaint in order to bring the older buffer up to
date with the last buffer we rendered into.

Rename to previous_damage so it's a bit more clear what this refers to.

Signed-off-by: Daniel Stone <daniels@collabora.com>