softpipe: fix depth testing problems

softpipe: fix depth testing problems

The optimized Z-test functions assumed that the array of incoming quads
are adjacent, but that's not always true.  The fragment shader can cull
intermediate quads, for example.

Now these Z-test functions can cope with non-adjacent quads.  A little bit
of performance is probably lost, but it's probably not worth worring about.

This fixes broken glBitmap() Z testing, among other things.

(cherry picked from commit d437d905e6924ebc05ec9efe87e1e2c48d75bc13)