GLX backend: fallback to lesser swap_control extensions Ideally we want to use EXT_swap_control as the code currently does as it allows us to specify the swap interval of each drawable independently. However open source drivers do not expose this extension and instead expose the MESA and SGI versions that have a global swap interval. This commit changes the code to use the best swap_control extension it can, potentially changing the swap interval on each swap buffers on lesser extensions. BUG=angleproject:1206 Change-Id: I251c6edf400aeeae5359a4370020e74b8e6cae51