[platforms/drm] Rewrite rotation rendering to use MapTexture

Summary:
The previous code uses a fixed version of GSLS which does not match our suported GL targets. We need both, using kwin's existing MapTexture framework we can achieve the same thing in a better way with less code.

Test Plan:
Had some slight issues with the rotation, but everything appeared the same
as when I applied the patchset with the custom shader.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26371