This is an ES2-only incompleteness check. We also need to require
matching dimensions in D3D11, but make this an implementation
specific check. Also make all implementation specific errors
'UNSUPPORTED' since that catches all "non-ES" framebuffer
restrictions.

Note that we can't be conformant here in D3D11 currently, since the
spec only makes an exception for mismatching formats for UNSUPPORTED,
not for size checks. However, we don't have an easy solution.

BUG=angleproject:1225

Change-Id: Ic80a04bce397fc12643b010c874f432033babc5d
Reviewed-on: https://chromium-review.googlesource.com/313990Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>