Add wgpu texture support (and windowing in Rust)

[dfellis]

For the demo of the language to have a bigger impact, it needs to be showy, and nothing is more showy than a graphical demo, especially if that demo is short, readable, and performant.

This is the very last task before attempting to draw attention to the project.

[dfellis]

Some information gathering for this.

I only want to support compute shaders (at first?) so I'll need to use copy_buffer_to_texture to actually render the pixels.

This requires some wrapper types for the buffer and the texture to work:

• ImageCopyBuffer where you provide the buffer reference and a layout describing how the raw bytes should be interpreted as pixels.
• ImageDataLayout is the struct with the layout information. This might be hardwired at first since I won't support any texture manipulation in the initial demo.
• ImageCopyTexture is more complicated, but I don't have to worry about it, because I can just have the Texture generate one for me with as_image_copy
• I can get the texture from winit (assuming I use that library, but probably since that's what wgpu uses in their examples, such as this example
• The window is evented, but Alan doesn't have that, so some way to automatically bridge that is going to be necessary. Most of these problems also apply to a browser canvas element, so I expect whatever I do here should apply there.

[dfellis]

Just updating that this is mostly done. The following things are left:

• Finish the "nice" API that eliminates the manual context buffer creation and access, the manual pixel memory calculation, and needing to worry about the RGBA value ordering (which is actually BGRA on every machine I've tested, but I also want to have this determined at shader construction time just in case there's a GPU/OS combo out there that does it differently).
• Figure out why there appears to be a memory leak when running on MacOS but not Window or Linux.
• Get the browser implementation done.