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Detect and fix up non-premultiplied cursor data

Michel Dänzer requested to merge daenzer/xf86-video-amdgpu:fixup-non-premultiplied-cursors into master

X server >= 1.18 already had code for this, but it only caught cases where some pixels have 0 for alpha and non-0 for a non-alpha component. Turns out some apps (e.g. the Civilization VI game) use non-premultiplied cursor data which doesn't have such pixels, but can still result in visual artifacts.

This uses the method suggested by Kamil in https://bugs.freedesktop.org/92309#c19: check for pixels where any colour component value is larger than the alpha value, which isn't possible with premultiplied alpha.

There can still be non-premultiplied data which won't be caught by this, but that should result in slightly incorrect colours and/or blending at the worst, not wildly incorrect colours such as shown in the bug report below.

v2:

  • Disable the check with current xserver Git master, which already does the same check now.

Bugzilla: https://bugs.freedesktop.org/108355 Suggested-by: Kamil Páral kamil.paral@gmail.com

Edited by Michel Dänzer

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