Bug 1) The offset wasn't plumbed through for setSubImage.
Bug 2) The DynamicBuffer allocation sometime allocates a bit more
than requested, but we were using the size requested as the next offset
instead of the actual allocated size. This could get us in a situation
in certain corner cases where the next allocation would be done on the
said buffer instead of using a new allocation as it should.

Also enables a bunch of new texture_specification_* tests that were
unable to run successfully without these 2 bug fixes.

Found a weird issue on WIN AMD only and suppressed these tests for now.
Will investigate part of the same bug number as a separate change.

Bug: angleproject:2495
Bug: angleproject:2492

Change-Id: I490b1bf2d1795b7a1033365e29eac12a8bc50bff
Reviewed-on: https://chromium-review.googlesource.com/1024380Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Luc Ferron <lucferron@chromium.org>