Fix unsized formats in GenerateMipmap. We had accidentally blocked mipmap generation for all unsized formats in ES3, because they were marked as non-renderable. The spec specifically allows for mipmaps with unsized formats in GenerateMipmap. BUG=angle:550 Change-Id: Iee7eddd011f4fe86b690422723620593f8049ba6 Reviewed-on: https://chromium-review.googlesource.com/186971 Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>