Fixed sRGB filtering by performing the conversion
pre-filtering when appropriate.

Change-Id: Id24898feed7e9a7dadff45431198771af25efed7
Reviewed-on: https://swiftshader-review.googlesource.com/15409Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>