In https://swiftshader-review.googlesource.com/c/SwiftShader/+/33028 we
modified the conversion of 8-bit color elements to 16-bit ones from
duplicating the byte, to putting 0 in the lower byte, and adjusting the
normalization factor from 0xFFFF to 0xFF00 accordingly. However, the
latter wasn't being done for B8G8R8A8.

Bug: chromium:979652
Bug: swiftshader:122
Change-Id: I621a600128e1b028b3303917acd826a7bf4630dd
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34228
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>