Simply using the ReadPixels code paths to unpack the framebuffer into a texture
was insufficient for handling cases like LUMA or RGB textures that are emulated
with RGBA textures in D3D11.  Instead, read the framebuffer's data into a
buffer and then use the loading functions which are aware of the emulated
formats to write the data to the destionation.

Fix LUMA format structure's logic for reading colors.  They aren't supposed
to do any averaging logic.

BUG=angleproject:1095
BUG=483282

Change-Id: Iad91d193d4c824573d9e9cafd28cf456d2de8be5
Reviewed-on: https://chromium-review.googlesource.com/354482Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>