backends/drm: upload custom geometry instead of using glScissor for optimized rendering

Vlad Zahorodniirequested to merge
work/cherry-pick-80a91b1a into Plasma/6.2

Empirical data suggests this yields a decent reduction in GPU usage

CCBUG: 477223

(cherry picked from commit 80a91b1a)

Co-authored-by: Xaver Hugl xaver.hugl@gmail.com

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