Tested with ANGLE running dEQP-GLES2.*rbo_rgb5_a1*

Bug: b/139411772
Change-Id: I5681b008ae7bfb2df2705189f28667dac8d4c983
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35812Reviewed-by: Nicolas Capens <nicolas.capens@gmail.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>