rework testsuite

[Vipitis]

We mainly took the wgpu-py testsuite as is and kept the relevant tests and examples. Some new issues came up related to caching (unexpected objects from _default_device) and while digging around I found a bunch of redundant parts to the current pytest setup.
This will might be a longer process since the documentation seems kinda thin. I also open this issue for discussion on specifics.

some of the tasks I see right now

• investigate and fix caching issue
• remove unused code
• move conftest.py to pyproject.toml
• test screenshots against thumbnails (https://www.shadertoy.com/media/shaders/shaderID.jpg)
• test examples giving in README
• add an API key secret to the repository so CI covers the API tests.
• offscreen doesn't seem to catch all errors with just shader.show() - needs investigation since the whole test suite is offscreen
• pin specific dependency versions for CI/dev
• bulk test for compatibility, like Most common errors with the Shadertoy utility #15 but maybe using https://github.com/Vipitis/shadertoys-dataset