backends/drm: upload custom geometry instead of using glScissor for optimized rendering
Empirical data suggests this yields a decent reduction in GPU usage - on my laptop when moving a blurred konsole around fast with a complex ICC profile set, I get
- about 80% GPU usage on master without damage tracking
- about 70-75% GPU usage on master with damage tracking
- about 50-60% GPU usage with this + damage tracking enabled
It would be good if someone else could verify that this helps on their system as well.