Handle preUpdate ahead of updating material uniforms to avoid 1 frame latency

[mrxz]

Fixes #390

The preUpdate flag is supposed to ensure the splat update takes place before rendering. While this was technically true, the updated splats were not used in the rendering as the relevant uniforms had already been set. This also meant that the uniforms referred to values from an accumulator that had already been released.

This PR moves the updateInternal call to the start of the onBeforeRendering method. In case of preUpdate this would ensure a synchronous update before the uniforms were updated. Whereas the default case is still to schedule the update asynchronously through setTimeout.

Note that whether or not the result of update is visible that same frame still depends on whether or not the mappings are compatible, though in simple cases like recolor this holds.