[Visual] VisualModelImpl: add data to optionally generate uv coords

[fredroy]

... using UV spherical projection.

This was done automatically if there was no texcoords previously (most certainly from a mesh).
But more often than not, the texcoords are not used so this is wasteful.

same as the previous PR, this is "functionally" breaking because it changes an implicit behavior to an explicit one (with an opposite default value), but I dont want to nag with message like "it was generating texcoords before, if you want to to do it set the data blabalbla"

[with-all-tests]

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