Bug 947847. Avoid reseting to empty rects. r=BenWa

pixman_region32_reset will give an error if the rectangle you pass to it is
empty. We can just check before hand if the rect is empty and in that case just
call pixman_region32_clear instead of pixman_region32_reset.

--HG--
extra : rebase_source : dbdde170f391b6e5c8808717039677b026adb701