Implement early depth test.

Implement early depth test.

Early depth test is enabled when depth test is enabled and
alpha test is disabled and fragment shader does not write
depth.
The early-z is implemented by moving the depth test stage
just before the fragment shader stage and prepending it
with an earlyz stage, introduced with this commit.
The earlyz stage prepares the quad->outputs.depth for
the following depth test stage by interpolating Z position,
just as the fragment shader would do.