Use R and RG textures with swizzle states to emulate them. A manual blit is
required when calling CopyTex[Sub]Image from a RGB[A] framebuffer to an
emulated L[A] texture so that the alpha channel ends up in the correct channel
of the destination texture.

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

BUG=angleproject:1113

Change-Id: If5540e66d9017596bd83d95ec3ede043cbcfe0d2
Reviewed-on: https://chromium-review.googlesource.com/293905Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>