dix: Work around non-premultiplied ARGB cursor data harder
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.