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.

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