This is a partial fix for 3455, which contains a number of different
root-causes.  This CL addresses two sets of them.

1) Partial fix for KHR-GLES3.copy_tex_image_conversions.forbidden.*

   Recent changes, including adding support for GL_RGBA8-to-GL_RGB5_A1,
   plus adding support for GL_RGB10_A2-to-GL_RGB5_A1 eliminate the
   crashes for 56 of the 70 tests.

2) Most of the KHR-GLES3.packed_pixels.rectangle.r* tests now pass.

   Recent changes, including potentially the above get another 34
   tests passing, and another 15 no longer crashing.

3) Vulkan: Fix errant glCopyTextImage validation code.

   I got all but 2 of the KHR-GLES3.packed_pixels.rectangle.r* tests passing.

   I fixed bugs in the validation code (with at least two more bugs
   that I haven't fixed yet).  More details below.

3a) Fix the IsValidES3CopyTexImageCombination() function for SNORM.

    The code wasn't rejecting SNORM cases, which are missing from the table of
    effective internal formats, meaning that support is undefined.  The
    GLES 3.2 spec says that a GL_INVALID_OPERATION error should be generated
    "if the effective internal format of the source buffer does not match the
    effective internal format of the new image."

3b) There's a caveat that was overlooked that means that internal
    formats like GL_RGB10_A2 are not supported.

Bug: angleproject:3455
Bug: angleproject:3693
Bug: angleproject:3697
Change-Id: Ie4399a2d7cd969ec29acc926f32e6608775609c6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1693325Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Ian Elliott <ianelliott@google.com>