Buffer mapping example

[almarklein]

Following up on #305 I created an example that demonstrates behavior related to buffer mapping that may or may not be expected.

As an experiment, I wrote this in Python, not via wgpu-py, but only using cffi as an interface. If this works for the other devs, this could be a nice way to quickly produce examples like these, and also interact with the lib interactively (e.g. in a Jupyter notebook). Otherwise I'll translate it to a C example :)