FramebufferGL: Fix blit workaround corner case
FramebufferGL: Fix blit workaround corner case

The SRGB blit workaround had to wrong assumptions:
 - SRGB blits can have a multisample source.
 - The woarkound is needed even when the filter is GL_LINEAR in the
   case where we are doing a RGB -> SRGB or RGB -> SRGB blit.

BUG=angleproject:1492
BUG=chromium:658898

Change-Id: I1d89572565a4e23c1c97bdf985bb21a445e898b7
Reviewed-on: https://chromium-review.googlesource.com/409540
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Commit-Queue: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>

MSOpenTech/angle,ppy/angle,MSOpenTech/angle,MSOpenTech/angle,ecoal95/angle,ppy/angle,ecoal95/angle,ecoal95/angle,ecoal95/angle,ecoal95/angle,ppy/angle,ppy/angle,MSOpenTech/angle