Setting Scene#skyBoxRotation invalidates all shader variants

[willeastcott]

For the 3D Tiles project, this causes about 80% of a frame to be spent on regenerating shaders:

I can work around this by now calling Scene#skyBoxRotation every frame, but this engine behavior is currently way to aggressive.

[mvaligursky]

Note to myself (or whoever will fix this): set skyboxRotation function calls _resetSky which sets updateShaders = true;. This is used to regenerate shaders that depend on skybox rotation. I think if the matrix is identity, the shader chunk is skipped. This should be matched here and resetSky should perhaps be only called when the matrix switches between identity and non-identity.