Bug 973976 - Remove redundant work in CopyableCanvasLayer. r=jrmuizel

This operator was already initialized to OP_SOURCE, so no need to do it again.
Bug 973976 - Revert to Thebes path for CopyableCanvasLayer. r=jmuizelaar

The performance on the Mac platform degraded after porting the code to
Moz2D in Bug 948765. This patch chooses the old thebes path instead of the
Moz2D path, so that performance is unaffected. This way we can easily
revert this patch at a later time when the perf issue has been fixed.
Bug 982183 - Put VTTRegion DOM API behind a pref. r=rillian, r=bz
Bug 982694 - GTK3 - Ensure that we build with system-cairo. r=ted
Bug 982960 - Add GLContext support for draw_range_elements. r=kamidphish
Bug 982960 - Use glDrawRangeElements in WebGL.drawElements. r=kamidphish