When sampling from an incomplete texture, the OpenGL spec states that
(0, 0, 0, 1) will be returned to the shader from the texture lookup
function. Swizzling is only described for filtered texture values.

Change-Id: Ie8a02b3b0b9e571fd2f86302e777a4ff985da022
Reviewed-on: https://swiftshader-review.googlesource.com/17488Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>