Support shader channels

[sin-ack]

A majority of Shadertoy shaders use channels to provide textures to the shader. We cannot run these shaders unless the texture is provided somehow.

Open questions:

• How to provide the texture? CLI argument? Have a folder represent a shader?
• Shader texture format? Pull in some library that can handle multiple formats? Don't wanna bring in too many dependencies. Maybe zigimg is good enough? Look into how Shadertoy itself does it.
• Channels are not only for providing textures, but for sound, sub-shaders etc. as well, including stuff like microphone input (!). How do we represent this? Maybe there should be a descriptor file (ZON makes sense I guess) that says what each channel is about.

[nailuj05]

Would like to see this too!

Would prefer having a folder define the shader. Maybe with a config file to assign resources to channels. I don't have much zig experience but from my reading it seems like zigimg doesn't fully support jpg, which might be a dealbreaker. (Maybe look into stb_image.h?)

[sin-ack]

Yup, zigimg has issues with the JPG decoder. I know because I wrote it ;) I'll look into our options.