add support for stencils

[tesselode]

Acknowledgements:

• @PsichiX for helping me get stencils working
• @LPGhatguy for suggesting a StencilState struct
• @lunabunn for design feedback

[17cupsofcoffee]

Really happy with this API! Seems to give all the low level control people would need without being super verbose (think the StencilState methods help a lot with that).

More importantly, I can kinda understand what the stencil example is doing, despite not being very knowledgable about OpenGL stencils, which is a good sign, I think.

As discussed yesterday, my only real concern is the change to Canvas::new (due to it being a breaking change, and the fact that it adds extra typing/imports even when you're not using this feature). For now, I'll probably tweak the changes to be non-breaking, and then revisit that when I come to do 0.7.

[17cupsofcoffee]

Thank you again for the PR! 🚀

[tesselode]

of course!

[17cupsofcoffee]

FYI - I just pushed a commit changing CanvasSettings into a builder, as otherwise it wouldn't be possible to add extra settings (e.g. depth buffers, texture formats, mipmaps) without each one being a breaking change. Should still be basically the same amount of typing (minus one import, which is kinda nice), but a bit more future proof.