On some GL driver/window system combinations the default framebuffer may
do SRGB conversion, which affects the values written there by draw
operations and read by readPixels. This is known to happen at least on
some Intel systems. For this reason, use an FBO with a known format
instead in the texture end2end tests.

This enables turning on most TextureTest cases on the OpenGL backend.
Tested on Intel HD 4600.

BUG=angleproject:1261
TEST=angle_end2end_tests

Change-Id: I5a39bd1dadf15b7c52793a0f66d67e9f61618b83
Reviewed-on: https://chromium-review.googlesource.com/321612Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tryjob-Request: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>