Fix CpuMesh.albedo_factor overwriting

[EtaLoop]

What

If Asset3D.albedo_factor isn't specified, CpuMesh.albedo_factor will always be set to re_renderer::Rgba::WHITE.
However, in some cases CpuMesh.albedo_factor may contain a color that we don't want to be overwritten.

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[Wumpf]

oops! Should have noticed this in the PR review. Thanks for following up! While we're on it, can you please also fix the other bug pointed out in the comment