Show depth buffer

[almarklein]

This is a bit of an experiment to what we'd need to make the values of the depth buffer available (for users, via an API, and for post-processing). It also contains a few tweaks to the blender code to avoid resetting the depth buffer in transparent passes.

For most examples, this nicely shows the depth:

Some observations:

• Transparent fragments do not have a depth value, unless the blend_mode is "opaque" or "ordered1".
• For subplots, only the last drawn plot has a depth, because the full depth buffer is reset for each subplot.
• Similarly, for overlays, only the depth of the final pass is visible.