GLX backend: fallback to lesser swap_control extensions Reland with a compilation fix. We would like to use the EXT_swap_control extension 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: I6c24217c7cfd5e09f60469cc7d6336cd68bc5c32 Reviewed-on: https://chromium-review.googlesource.com/312270 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tryjob-Request: Jamie Madill <jmadill@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>