Small refactors

Xaver Huglrequested to merge
work/zamundaaa/some-refactors into master

Commit 1: platformsupport/scenes/opengl: filter out external formats properly

Commit 2: backends/drm: remove egl config handling. It's not needed anymore

Commit 3: backends/drm: remove unused variable

Edited by Xaver Hugl

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