Fixed R8G8_SRGB and R8G8B8_SRGB in the Blitter.
Enabled R8G8_SRGB tests to make sure the fix is correct.

Change-Id: Ibe7addd3f635f353abb09e0391a9a30d20392fbf
Tests: dEQP-VK.*r8g8_srgb*
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33530
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>