Some internal refactoring, fix the doc theme, and compatibility with rendercanvas.
Changed:
* Do not confound STDERR and STDOUT by hmaarrfk in https://github.com/pygfx/wgpu-py/pull/638
* More consolidation by fyellin in https://github.com/pygfx/wgpu-py/pull/641
* Use rtd theme for docs by almarklein in https://github.com/pygfx/wgpu-py/pull/639
* Refactor present-method mechanic by almarklein in https://github.com/pygfx/wgpu-py/pull/642