Rasterizer discard implementation

Implemented rasterizer discard by not using the SetupRoutine
and always returning 0 when rasterizer discard is enabled.
Also guarded all clear calls with a check for rasterizer
discard. Passes all rasterizer discard related dEQP tests.

Change-Id: I1e5c107e3dba550f7a5b01eb302ff51bdac303dc
Reviewed-on: https://swiftshader-review.googlesource.com/4876
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>