Patched on top on 79b0e13913b5189bb8629e80439fea746f99fe79 and it works! Many
thanks!

Machine used is a PowerMac G5 7,3 with an AGP Radeon 9600 Pro (Mac edition).
Running Compton composting manager (Ubuntu MATE 16.04) and it runs without
glitches so far, colours look good. Only thing I noticed is the desktop gets
unresponsive and the mouse pointer very jerky during youtube video playback.

Will try Piglit tests the next days.