[FEATURE] Supporting vertex colors for gltf loading

[paireks]

Hey @xeolabs and @MichalDybizbanskiCreoox!

I wanted to add vertex color support to our gltf loader. This PR is adding it, so we can load it like this:

The thing I cannot figure out is loading existing HousePlan file. After a change it looks like this:

When I debugged it, it looks like this model contains the vertex colors, that are all (1,1,1,1) -> (which is all white), that's why it changed this color to white. Some other models don't have that, they just don't have vertex colors. I don't think ignoring vertex colors when they are all 1,1,1,1 is a good approach. Please let me know if you have an idea how to handle this.

Also, if you will find anything else during review: please let me know.

This topic is connected with XCD-273.

[paireks]

Ok, after reading a bit more of documentation and experiments I think I get it a bit more, hope to discuss it tomorrow with you

[paireks]

So after a change (as the specification suggests) there is a mutiplication of a base color by vertex colors multipliers. It is a bit of a workaround to store this result of multiplication inside the ".colors", but this way the existing models should look same as before, even if they had these multipliers [1,1,1,1] like here:

Plus I added an example of a box with a base color AND vertex colors, and after this change it looks similiar to the existing viewers (because of this multiplication):