gl-renderer: track damage on dummy renderbuffers

Create dummy renderbuffers to track surface buffer damage on
demand. The renderbuffer representing the surface buffer that
is currently rendered to is inferred using buffer age.
This aligns damage tracking with the Pixman renderer and will
simplify adding FBO rendering support.

The previous/total_damage regions and the buffer/border_damage
arrays are now unused except for validation.
They can be removed next.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>