Better Pyglet integration

[wadetb]

standalone_pyglet.py

I threw this together (see PygletGLRenderer) because I didn't want to bring in PyOpenGL when Pyglet already offers a GL binding.

Not well tested, and doesn't take advantage of all the high level Pyglet stuff (I couldn't figure out how to anyway) - but it at least brings up a window on my Linux machine without depending on PyOpenGL or glfw.

LMK if you would like a PR and how to organize it, or feel free to incorporate it yourself, or just ignore it.

[nurpax]

Hi @wadetb! Thanks, fewer deps is always a good thing so this is a great improvement for Pyglet users.

If you have the time and motivation to craft this into a PR, I'd be happy to review and merge. I looked at the code and it looks fine. IMO it'd make sense to move the renderer (both GL and pyglet I/O stuff) into pyglet.py under src/slimgui/integrations/ and then adapt example/standalone_pyglet.py to use that.

There's DEVELOPMENT.md that explains how to build/install from source. It needs to use that bash script instead of just pip install because of reasons.. Only problem is that it hardcodes one path for macOS. (it should be possible to change at least locally for Linux)