swr/rast: Fix early z / query interaction

For certain cases, we perform early z for optimization. The GL_SAMPLES_PASSED
query was providing erroneous results because we were counting the number
of samples passed before the fragment shader, which did not work if the
fragment shader contained a discard.

Account properly for discard and early z, by anding the zpass mask with
the post fragment shader active mask, after the fragment shader.

Fixes the following piglit tests:
    - occlusion-query-discard
    - occlusion_query_meta_fragments

Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>

src/gallium/drivers/swr/rasterizer/core/backend.cpp

@@ -593,6 +593,10 @@ void BackendSingleSample(DRAW_CONTEXT *pDC, uint32_t workerId, uint32_t x, uint3
                             pDepthBuffer, depthPassMask, vCoverageMask, pStencilBuffer, stencilPassMask);
                         goto Endtile;
                     }
+                } else {
+                    // for early z, consolidate discards from shader
+                    // into depthPassMask
+                    depthPassMask = _simd_and_ps(depthPassMask, vCoverageMask);
                 }
 
                 uint32_t statMask = _simd_movemask_ps(depthPassMask);