Relax detection of non-premultiplied alpha cursor data

The stricter detection broke the cursor in some games. Apparently those use cursor data with premultiplied alpha, but with some pixels having r/g/b values larger than the alpha value (which corresponds to original r/g/b values > 1.0), triggering the workaround.

Relax the detection to match what's in the X server since 1.18.4, but keep the workaround for older versions.

Bugzilla: https://bugs.freedesktop.org/108650