Uses 0, -1 offsets instead of 0, 1 to be able to use it as a
mask. Optimize calculation of the integer coordinates and fraction by
always having xyz1 = xyz0 + 1 (before wrap). Optimize the
clamping/wrapping of the integer coordinates. Skip addressing operation
of the third component for cube sampling (since already projected to
2D face).

Change-Id: If2e7c74aac9ae923a0c1ffc278fcdfec00f216f3
Reviewed-on: https://swiftshader-review.googlesource.com/14269Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>