mesa: Add unpack_uint_z_row support for floating-point depth buffers

mesa: Add unpack_uint_z_row support for floating-point depth buffers

This is a hack, and it will result in incorrect rendering.  However,
it does eliminate spurious warnings in several piglit CopyPixels tests
that involve floating-point depth buffers.

The real solution is to add a zf field to SWspan to store float Z
values.  When a float depth buffer is involved, swrast should also
populate the zf field.  I'll consider this post-8.0 work.

NOTE: This is a candidate for the 8.0 branch.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>