Feature request: Custom attributes

[spacejack]

Would be great to have something like three.js's BufferGeometry that allows us to use custom attributes for our own shaders.

Additionally, InstancedBufferGeometry as a way to use instancing.

There is also InterleavedBuffer and InstancedInterleavedBuffer, perhaps these should also be considered?

[vitvakatu]

Blocked by gfx-hal support.
@spacejack for your information, Three already supports instancing (see Factory::mesh_instance method)

[kvark]

@vitvakatu I don't think this is blocked by HAL. Created #175 to discuss this separately.

We should find a way to extend the custom shader support with:

• a single user-provided vertex buffer
• (option) a single user-provided instance buffer (can be empty)
• a single user-provided constant buffer

This is fairly limited for sure, but we aren't aiming for the full-blown customization.