Simplify GenerateMipmap validation now that sized format info is tracked. 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>