Changes gl::RenderbufferStorage to rx::RenderbufferImpl

BUG=angle:782

Also eliminates Colorbuffer/DepthStencilBuffer and related friends
and adds the RenderbufferD3D implementation of RenderbufferImpl.

Change-Id: Ibe72c9589b3c238b952744e7cfd70d97ec898eb9
Reviewed-on: https://chromium-review.googlesource.com/222925
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Shannon Woods <shannonwoods@chromium.org>
Reject shaders of a version that the compiler doesn't support.

For example, WebGL 1.0 compiler should not compile WebGL 2.0 shaders.

BUG=angle:
TEST=815

Change-Id: Iaec66fa5127426778aff6e3891e2c214048d7457
Reviewed-on: https://chromium-review.googlesource.com/226380
Tested-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Changed feature macros to reduce accidental enabling/disabling

Change-Id: Ieb1a0c55f412f2a1bb858522b2dfaa7a60aa7ddb
Reviewed-on: https://chromium-review.googlesource.com/226304
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Brandon Jones <bajones@chromium.org>
Fix angle_unittests failures.

BUG=angle:815
TEST=angle_unittests

Change-Id: I4d59f227b7c72f4f456d0a60d8507d097ef05dec
Reviewed-on: https://chromium-review.googlesource.com/226463
Tested-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>