FramebufferGL: only work around SRGB blits on Desktop GL BUG=angleproject:1492 Change-Id: I6c210243dffb775efd61c33160dabcbf7406a757 Reviewed-on: https://chromium-review.googlesource.com/397798 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>