Note that generate mipmaps is not yet supported, but uploading mipmaps
is. This also enables 64 tests in dEQP gles2 to validate the changes.

While trying to enable tests in functional.texture.mipmap.*, I found
an issue where the graph ends up in an invalid state and triggers and
assert in onReadResource in ContextVk.cpp:188. It seems like an
unrelated bug to mip maps, I will investigate to enable these tests
separately in a following CL.

Bug: angleproject:2479

Change-Id: If51776d8ef3d994bee620d6a1cf982bb51838ff0
Reviewed-on: https://chromium-review.googlesource.com/1022232
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>