Instead of trying to encapsulate the GL texture format workarounds into
tables, use functions that do manual checks for specific cases.  Simplifies
the logic.

Fixes:
 * conformance/extensions/ext-sRGB.html
 * conformance/extensions/oes-texture-half-float.html
 * conformance/extensions/oes-texture-half-float-with-canvas.html
 * conformance/extensions/oes-texture-half-float-with-image.htm
 * conformance/extensions/oes-texture-half-float-with-video.html

BUG=angleproject:884

Change-Id: Ifb719fff908680fddc7c53a544e2284a42a58356
Reviewed-on: https://chromium-review.googlesource.com/289082Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>