I found a good workaround. the issue is with cursor.png.
I edited the cursor.png with gimp, and paint the "transparent square" with
black and then save with "save color of transparent pixel" checked and now it
works !

attached the modified cursor.png

just copy it to SteamApps/common/Planetary
Annihilation/media/ui/main/game/live_game/img/cursors/cursor.png and voila

so for some reason mesa/xcursor or radeonsi think transparent pixels with white
color are not transparent. (red are partialy transparent, and black are
completely transparent).

I attached the two cursor.png file, so it can be tested/reproduced easily.