compositor: Correctly calculate changed sub image rectangle
compositor: Correctly calculate changed sub image rectangle

When calling glTexSubImage2D for sub image updates for SHM surfaces the
changed rectangle was being wrongly calculated. This resulted in interesting
redraw artefacts for clients using SHM.

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