* No validation that the format matched the texture level's format for
   SubImage calls.
 * WebGL does not allow the base level to be smaller than the block size.
 * ANGLE used to allow mips of size 3 when this is disallowed.
 * Don't early-exit validation when dimensions are 0, imageSize validation
   happens later.

TEST=conformance/extensions/webgl-compressed-texture-s3tc

BUG=angleproject:1998

Change-Id: I05f5a0b5180344d67b036fdecc17edd2256e85ab
Reviewed-on: https://chromium-review.googlesource.com/480442Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>