v0.5.0: revert PLY workflow lol! also fix player builds and other fixes

Previous release (v0.4.0) changed workflow to have a custom PLY importer, so you could just drop your gaussian splat file into Unity project and be done with it. Turns out, that does not work so well when there's some other package in the same project that wants to import PLY files (e.g. keijiro/Pcx), #15. So now we're back to having a menu item Tools/Gaussian Splats/Create GaussianSplatAsset. 😓

Other things in this release:

• Guard against splats occasionally "exploding" over whole screen due to degenerate covariance matrix (thx @pastasfuture)
• Fixed player builds, editor-only code was not properly excluded from them (thx @francescofugazzi)
• High Quality preset is actually lossless now, i.e. keeps everything in full 32-bit floats. It's slow and large, but useful if someday I'll build some sort of "export gaussian splat back" utilities.