Turns out some apps (e.g. the Civilization VI game) use
non-premultiplied cursor data which doesn't have any pixels with 0 alpha
but non-0 non-alpha, 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.
Bugzilla: https://bugs.freedesktop.org/108355
Suggested-by: Kamil Paral <kamil.paral@gmail.com>
b45c74f0f2