Expose custom vertex attributes to MeshDataTool

[lyuma]

Salvaged PR by @fire: Add support for custom vertex attributes to MeshDataTool
Color is used as an array of 4 floating point numbers.

[clayjohn]

It would be good to link to the original PR (#52214) in the description and make the original author a co-author as well

[bitsawer]

I wonder if it might be better to use Vector4 (now that we have it) or something else instead of Color for custom data. There is the real_t vs float issue, but conceptually Vector4 might be better here. Especially if we want to possibly support full double data types in shaders in the future for harware that supports them (at least for custom vertex data). aaronfranke had some good point about similar issues back when Vector4 was discussed: godotengine/godot-proposals#629 (comment)

[lyuma]

@clayjohn Thanks for finding the original PR. I'll add credit for the MeshDataTool apis, and as noted in the original PR, might be good to make sure the SurfaceTool API also lines up.

@bitsawer regarding Vector4 and real_t, shaders generally use 32-bit floats or halfs. I don't see a per-vertex double attribute being particularly well supported or used. If I compile Godot to doubles, i would not expect that to change all shader properties and vertex attributes to double internally.

I'm okay with using Vector4 if gets converted back to float in the end. Though there's a lack of consistency in Godot apis right now that makes this whole thing a bit awkward. Some use Color, some use PackedFloat32Array, some use Plane (shader parameters used Plane, unless this got changed) and some use Vector4. I would copy either what is used for tangents, or what is used for uniform vec4 shader parameters.

[unfa]

I'm waiting for this feature to be able to generate tangents for extra UV channels at import so I can then blend multiple normal maps (using different UV channels).