D3D11 supports sampling other channel sizes of backing object
and declaring them as 4-vectors in HLSL. This allows us to
simplify the logic for fast unpack shaders significantly.

Source:
http://msdn.microsoft.com/en-us/library/windows/desktop/bb509700(v=vs.85).aspx
(Default Values for Missing Components in a Texture)

Change-Id: I7f645372f266f57bd94cdb1c795f1d30bf2a60a4
Reviewed-on: https://chromium-review.googlesource.com/181901Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>