Added 3D brush outline shearing and fixed triangle selection

[Nestorboy]

• Rewrote how the brush outline is transformed by computing a custom matrix to allow for shearing.
• Added a function to mesh.js and cube.js to compute a texel to local matrix for the brush outline.
• Fixed issue where the brush uv position would get truncated before being transformed to a world position for the outline, causing it to select the wrong triangle.

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[Nestorboy]

@JannisX11 there's some code duplication in mesh.js where I compute the brush matrix from the bary coords. Should it be moved to some particular location or is it fine?

[JannisX11]

Yeah, you can use a closure inside the TexelToLocalMatrix function to handle that duplicated code. Also make sure to not modify the passed uv parameter directly, as it may still be used for something else after calling the function. TexelToLocalMatrix should be texelToLocalMatrix, following lower camel case.

I noticed two issues when testing this feature initially:

• Brush outlines no longer work on cubes
• Higher pixel density textures no longer work, the brush outline displays too large. Texture resolutions can be higher than the UV resolution, and the brush outline should support that.

[Nestorboy]

Thank you Jannis for taking the time to test this and giving me some pointers and advice regarding my code!

I've gone over and fixed the issues you discovered, and realized that there were some other small code cleanups I could do. At this point I believe it's ready to be reviewed and merged so I'll turn it into a regular PR instead of a draft.

GitHub Desktop seems to only have let me create a new branch from master, so I accidentally included a commit from master branch which next doesn't have. It might not matter much, but would you mind syncing the next branch?

[Nestorboy]

I realized I never accounted for the soft brush not doing any form of snapping, so I've fixed that too. I also made the code behave more similarly to the previous implementation where the truncation was done before the UVToLocal function