dEQP is very unhappy (and asserts if built in debug) with this since it
has the wrong channel count.

Test: dEQP-GLES3.functional.read_pixels.alignment.*
Change-Id: I35a46ba166d3964091d5e3b3066e3a593952c22a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/36810Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>