Refactors the gl::Texture class and a few related methods. Also reduces
binary size by up to 4k.

Bug: angleproject:2491
Change-Id: Ib9a69d7f507b0dce35abb17b90532f812bf43214
Reviewed-on: https://chromium-review.googlesource.com/c/1291845
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>