This is done to make it possible to change some of the DXGI formats
of integer textures without affecting their color read function
associations.

The packPixels function gets the ANGLE format from the texture helper,
which now requires the ANGLE format to be passed in at the time of
creation.

BUG=angleproject:1244
TEST=angle_end2end_tests,
     dEQP-GLES3.functional.fbo.* (no regressions),
     dEQP-GLES3.functional.pbo.* (all pass)

Change-Id: I368337cfe5f8c86ff3292009ccf29e9d01409a07
Reviewed-on: https://chromium-review.googlesource.com/329213Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>