Previously we would store the unsized 'base' format as the internal
format. For instance for GL_COMPRESSED_RGB_S3TC_DXT1_EXT we would store
GL_RGB. With the new spec validation for TexSubImage it becomes clearer
to store the internal format of the compressed texture as the 'format'.

Bug: angleproject:3170
Change-Id: I7446418896eabd3d4f143e9cae4976889b973674
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1553379Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>