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