Example 19-fixedzoom scale not fixed initially

[julianoes]

Describe the bug

When I first start the example 19-fixedzoom, the scale is not fixed initially. It only starts working once I deselect pan or change something else. I think that's because plane.fixedZoom is never set until changeScale() is called.

To Reproduce
Steps to reproduce the behavior:

1. Run example 19
2. Try to zoom

Relevant Code or Code Sandbox

threebox/examples/19-fixedzoom.html

Lines 147 to 148 in b5473d7

	function changeScale() {
	plane.fixedZoom = (api.fixedZoom ? api.maxZoom : null);