global min-max settings

[burritojustice]

While there are advantages to having a dynamic color range based on the min and max values for a property that happen to be in view, there are times where it will be desirable or necessary to use a "global" min and max for the entire dataset.

Currently, we can only track this by keeping track of what the viewport has seen, but in the future the XYZ API /statistics end point may be able to deliver this.

[burritojustice]

note that we can manually override the min and max in the sigma controls