Since filerability and renderability may depend on separate extensions,
use functions to determine their support.  Context now uses these
functions to fill it's own TextureFormatCaps.

BUG=angle:658

Change-Id: Ib4dc877ba7c24f84a3823fd5aa2e01a3b3621705
Reviewed-on: https://chromium-review.googlesource.com/206831Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>