* Non-4 component formats cannot be used as swizzle formats.
* Unsized formats should not have swizzle formats.

BUG=angle:721

Change-Id: Ic4e2642f5dc4c9811768961fbab00f6edd200ce1
Reviewed-on: https://chromium-review.googlesource.com/211241Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>