This ensures that if the sample or layer coordinate were not forced to
be in range but the 'valid' mask indicates whether they're in range,
we sample texel 0 instead of potentially outside the image memory.

Note this does not yet validate the sample coordinate.

Bug: b/162419571
Change-Id: I08561862cbf371c1109c8949cd90557110d4ee24
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/47228
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>