The previous validation used some workarounds because it didn't know if
the texture format was sized or not.  Now that the InternalFormat struct
tracks this, the validation can work correctly for floating point
formats.

BUG=angleproject:2149

Change-Id: I583db4a36137a57dd1b7fc81cd4e3b6d5972fc67
Reviewed-on: https://chromium-review.googlesource.com/665163
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>