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glamor: Don't override source alpha to 1.0 if it's used for blending

Michel Dänzer requested to merge daenzer/xserver:glamor_composite_choose_shader-source-alpha into master

It caused an incorrect result of the blend operation.

Use glColorMask to prevent non-1.0 alpha channel values in a depth 32 pixmap backing an effective depth 24 window. For blending operations, the expectation is that the destination drawable contains valid pixel values, so the alpha channel should already be 1.0.

Fixes: d1f14289 ("glamor: Ignore destination alpha as necessary for composite operation") Issue: https://gitlab.gnome.org/GNOME/mutter/-/issues/3104

Edited by Michel Dänzer

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