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: I974a55fe3acc77ac77e93f19c83ee3b76f784df2
Reviewed-on: https://chromium-review.googlesource.com/302336Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>