softpipe: add support for explicit early depth testing

softpipe: add support for explicit early depth testing

ARB_shader_image_load_store adds support for explicit early
depth testing. However we need to make sure we don't overwrite
values using the shader written values in this case.

This fixes early depth testing in softpipe to conform with
those requirements.

Reviewed-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>