Add EGL_ANGLE_flexible_surface_compatibility This extension allows users of ANGLE to make contexts current with surfaces that were created with different bit depths. BUG=angleproject:1223 Change-Id: I9072c256e0a36aee1ce41ab69cb9a04240bf0521 Reviewed-on: https://chromium-review.googlesource.com/314935 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tryjob-Request: John Bauman <jbauman@chromium.org> Tested-by: John Bauman <jbauman@chromium.org>