Instead of querying DXGI format info to determine whether framebuffer
data can be copied to packPixels target memory with memcpy, compare
the internal format information from the ANGLE format to the packing
format. This makes it possible to change some of the DXGI formats
used for integer textures without breaking packPixels.

This makes it possible to entirely remove the DXGI format to GL
internal format mapping from the DXGI format info table.

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

Change-Id: Id5ffd581349e7afe39872d69cb05381f3e78d352
Reviewed-on: https://chromium-review.googlesource.com/329214Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>