Group all solid triangles independently of color

[hecrj]

This PR enhances our triangle pipelines to batch all solid geometry independently of color.

As a result, all solid geometry drawn sequentially in a Canvas will be batched together into a single draw call independently of color. The previous behavior produced as many draw calls as different colors used.

We achieve this by introducing a new ColoredVertex2D type and separating Mesh into SolidMesh and GradientMesh variants in Primitive.

No user-facing APIs have changed.

[bungoboingo]

Changes seem good, thanks for doing this! Tested on macos M1 glow & wgpu (only computer I have access to at the moment), so cannot speak to old GL versions but I don't see any changes that would break anything there. Also tested WASM build and seems good.

Only nit I have re: nesting solid & gradient mods in the triangle.rs. I found it easier to read when they were separated out into their own files. Realize this is subjective so don't feel particularly strongly about it.

[bungoboingo]

Can also pull in the old Geometry example again from 0.4 branch since we have colors in vertex data again (happy to do this after the merge!)

[artursapek]

did we mention it's fast

[bungoboingo]

Unless we want to add those From<Color> & From<Gradient>s in for canvas::style I'd say this is good to go.

[hecrj]

Only nit I have re: nesting solid & gradient mods in the triangle.rs. I found it easier to read when they were separated out into their own files. Realize this is subjective so don't feel particularly strongly about it.

I know it's pretty subjective, but I felt like the boundaries of the modules were a bit blurry (e.g. only some fields were public, methods needed to be called in a particular order and with correct indices, etc.). I usually prefer no boundaries at all than blurry ones.

We could still move the modules to their own files but, since everything is pretty coupled, I feel the pipelines are easier to edit this way.