Make members of SplatEncoding non-optional to avoid repeated fallback handling

[mrxz]

The SplatEncoding type was used both as the actual encoding values as well as the type for specifying non-default encoding values. This meant that each member had to be optional, which in turn meant a lot of code repeated the same default fallback values as the type suggested it could be undefined.

In practice however, these values would always be set and these fallbacks were just repeated to appease the type system. This PR changes the typing of SplatEncoding such that all members are required, removing the need for implementing fallbacks throughout the code base. For the PackedSplats and SplatMesh constructors a Partial<SplatEncoding> is used so these can still receive only the encoding properties that need to be overridden.

Additionally this PR contains some tightening of a few "result" types, marking their members as readonly for good measure, as well as the removal of two unused constants USE_COMPILED_PARSER_FUNCTION (obsolete with #374) and WASM_SPLAT_SORT (obsolete with #322)

[asundqui]

@mrxz this is a long overdue cleanup, I agree this is much better than all the optionals and fallbacks we had in that past. Thank you for your attention to it! @oscarlorentzon @dmarcos this is a good idea right?

[oscarlorentzon]

We might want to handle the remaining default assignments at PackedSplats.ts:1066 and SplatMesh.ts:1088. We can fallback with?? DEFAULT_SPLAT_ENCODING and then read the members off the object.

Should the (default: undefined) comments at PackedSplats.ts:88 and SplatMesh.ts:116 name DEFAULT_SPLAT_ENCODING now?

[mrxz]

We might want to handle the remaining default assignments at PackedSplats.ts:1066 and SplatMesh.ts:1088. We can fallback with?? DEFAULT_SPLAT_ENCODING and then read the members off the object.

Addressed both of these as well, does nicely get rid of some more repeated default values. Also eliminated them from the initial values of the uniforms, as the update method already takes care of setting it before rendering.

Should the (default: undefined) comments at PackedSplats.ts:88 and SplatMesh.ts:116 name DEFAULT_SPLAT_ENCODING now?

From the user's POV nothing should have changed. While it's slightly confusing that it states undefined, I'm not sure listing DEFAULT_SPLAT_ENCODING is going to make it clearer at this point. For one there's no reference to it in the docs currently and secondly the comment indicates it's to "override the default splat encoding ranges". In other words it would then imply that the default value for the override of the default splat encoding ranges is the default splat encoding values.

I'd rather leave the docs improvements for future PRs. Ideally we'd move to JSDoc based comments throughout, ensuring people get these comments straight in their IDE, and in time hopefully automatically generate the relevant (reference) docs from it.

[oscarlorentzon]

Leaving the docs as is for now makes sense. Looks good to merge @dmarcos.

[dmarcos]

thanks