Option to include gizmos in canvas snapshots

[xeolabs]

Problem

Sometimes we want to capture things like the SectionPlanesPlugin editing gizmo in canvas snapshots we capture with Scene#getSnapshot.

This is currently needed by the FastNavPlugin``` added for #601, which needs to include the SectionPlanesPlugin```` editing gizmo in the snapshot that it uses for its fade transition effect.

Solution

Add a includeGizmos boolean option to Scene#getSnapshot, which is false by default.

Usage

myViewer.scene.getSnapshot({
    includeGizmos: true
});

• [Run example]