Greatly simplifies the format utils by not having any internal format <->
DXGI format conversions depend on the client version. 32 bit floats and
24bit integers have very close precision in the [0,1] range so there
should be no precision issues.

BUG=angle:659

Change-Id: Ife64445ef35f77283fc4c70c98754f5e87b6fb19
Reviewed-on: https://chromium-review.googlesource.com/202507Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>