In DesktopGL, SRGB blending must be enabled or linear blending will be
used.

reland: Work around issues on AMD drivers where SRGB blending would be
used on clears of linear attachments.

Passes all dEQP-GLES3.functional.fragment_ops.blend.fbo_srgb.* tests (1106
new passing tests).

BUG=angleproject:883
BUG=angleproject:885

Change-Id: I03fbb0910ce414552ba841099a177446d0ace7b6
Reviewed-on: https://chromium-review.googlesource.com/301702Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>