Set viewport from query parameter(s)

[rod-glover]

Still deciding details. Two obvious candidates:

• viewport bounds
• center and radius

Questions:

• Should the qp(s) be updated by zooms by the user? Would be nice, but might be hard to do.
• Should the nominal (fully zoomed out, all Canada, looks fairly normal) aspect ratio needs to be preserved. If qp(s) are mutually updated, then this is effectively not possible, as zoom in can be at any aspect ratio, and the bounds option for the qp(s) is pretty much mandated.