This also exposed an issue in our glTexSubImage2D validation where the
sized format would be used for the texture support check when the unsized
format was provided.  The GL_ALPHA32F format is not valid unless
EXT_texture_storage is present but using GL_ALPHA + GL_FLOAT is valid.

BUG=angleproject:1958

Change-Id: Ice0b7549c39559990942176481c5175df17aaf92
Reviewed-on: https://chromium-review.googlesource.com/491246
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>