This CL exposes DXGI_FORMAT_R16G16B16A16_FLOAT as GL_RGBA16F.

Bug: angleproject:4440
Change-Id: Icbc7b2d10a4c02946fb01711c01cc8c9b01e8f54
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2079197Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>