Bug 1088414: Use a single synchronization texture for D3D11. r=jrmuizel

This patch adds a cross platform 'sync object' that is used to synchronize the drawing of individual textures. For the D3D11 implementation all textures that are written to will have one pixel copied into the D3D11 sync texture while holding its lock. The compositor will then, before composition acquire and release sync once, this should ensure all drawing on the content side has completed.