swr/rast: Fix early z / query interaction

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>