Basically, before drawing X-Rays, we now bind a second depth buffer.
After drawing XRays, we do an extra resolve pass where we overwrite the
non-XRay depth buffer in pixels where the depth is not maximum (which
means background pixel, since depth is cleared before drawing X-Ray
objects).
This ensures both scene and X-Rays keep their depth values and are ready
for compositing. Well, the odd effect due to depth discontinuities can be
expected, and X-Rays are a bit more expensive (extra buffer + resolve pass)
but at least X-Rays won't invalidate depth values anymore. Whee!
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