Fix zoom to mappable rectangle.

[na9da]

What this PR does

When zooming to a mappable's rectangle, use the same code path we have for Rectangle which is smarter and sets a minimum viewing distance instead of simply fitting the camera view to the rectangle. This is a better behaviour when zooming to simple shapes like a single polygon.

Test me

Before:

• Open this CI link for main branch and zoom to the item in workbench
• Observe that the zoom is too close and possibly confusing to the user

After:

• Open the CI link for this branch and repeat above steps
• Observe that the full layer is correctly in view

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.

[zoran995]

@na9da 2 tests are failing

• TerriaSpec initial zoom behaviour of initialCamera.focusWorkbenchItems zooms the map to focus on the workbench items
• TerriaSpec initial zoom behaviour of initialCamera.focusWorkbenchItems works correctly even when there is a delay in a Cesium/Leaflet viewer becoming available
  Others are not relevant and should pass after those 2 are fixed (they are flaky

Other than that, zoom looks much better now

[na9da]

Thanks @zoran995. Fixed. The spec was relying on the zoom happening immediately which was no longer happening because of the nested call.