Explicitly enable framebuffer SRGB blending in StateManagerGL. In DesktopGL, SRGB blending must be enabled or linear blending will be used. Passes all dEQP-GLES3.functional.fragment_ops.blend.fbo_srgb.* tests (1106 new passing tests). BUG=angleproject:883 BUG=angleproject:885 Change-Id: I0dfb744faa811f10be47c5bd8295b368baf3b04b Reviewed-on: https://chromium-review.googlesource.com/298620 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>