Use R and RG textures with swizzle states to emulate them.

Fixes the following tests when using the core profile:
 * conformance/extensions/oes-texture-float.html
 * conformance/extensions/oes-texture-half-float.html
 * conformance/textures/misc/tex-sub-image-2d.html
 * conformance/textures/misc/texture-formats-test.html
 * conformance/textures/misc/texture-npot.html
 * conformance/more/functions/copyTexImage2D.html
 * conformance/more/functions/copyTexSubImage2D.html

BUG=angleproject:1113

Change-Id: Iaf4b7b2b0000052b1747f46d90ad45d4cb1f6b50
Reviewed-on: https://chromium-review.googlesource.com/293530Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>