compositor: Don't use glClear for clearing
compositor: Don't use glClear for clearing

Draw solid color rectangles clipped to the repaint area instead.

eyolfson/weston,mchalupa/weston,jonnylamb/weston,Gnurou/weston,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present,giucam/weston,mchalupa/weston,Fantu/compositor-spice,kwm81/weston,jonnylamb/weston,krezovic/weston,krezovic/weston,Tarnyko/weston-xdg_surface_present,sir-murray/weston,Fantu/compositor-spice,udoprog/weston,udoprog/weston,giucam/weston,eyolfson/weston,Gnurou/weston,xorgy/weston,kwm81/weston,xorgy/weston,sir-murray/weston