Additional cfg for wasm target for gradients

[bungoboingo]

Storage buffers aren't supported by wgpu WASM + webgl which gradients currently use. For now I've just added a #cfg around relevant bits of code so WASM targets that don't use gradients will compile as expected.

Not quite sure if there is a preferred way to separate out this code, but I figured writing a separate implementation for WASM which uses a static uniform array like the GL target would be a relatively simple process so it didn't make sense to reorganize the code to reduce the number of target-specific compilation exclusions.

[tarkah]

Since our canvas API is already limited due to the opengl implementation, should we just use fixed # stops / uniform data on wgpu? I think a single implementation that is compatible with all targets is preferable to a bunch of code split. I really don't think it's an issue since hitting the max 16 color stops is such a far out edge case.

We can document the limitation on the canvas API and ensure the builder outputs None if more than limit was added.

[hecrj]

Looks good! Thanks!

[bungoboingo]

Yeah, I could reimplement this for wgpu with a fix number of color stops for a WASM target. As far as I can tell, WebGL cannot (or will not?) support SSBOs, so this will always be a limitation with a WASM target w/ wgpu's WebGL feature. I think instead of returning Err from the builder I would just ignore any stops after the limit with proper documentation.