The blitter was not doing the sRGB conversion on the first of
N source samples when accumulating samples for doing an sRGB
resolve.

This cl also includes Chris' initial fix for PixelRoutine and
SpirvShader. See:
https://swiftshader-review.googlesource.com/c/SwiftShader/+/31928

Change-Id: I97ce06fb39788fd623eb9b5c0203e45fd911c1dd
Tests: dEQP-VK.renderpass.suballocation.multisample.*.samples_4
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32848Tested-by: Alexis Hétu <sugoi@google.com>
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>