Bug: NiiVue deafults override FreeBrowse defaults when loading a niivue document

[pwighton]

FreeBrowse has certain defaults, like:

• crosshairColor: [1.0, 0.88, 0.88, 1.0]
• crosshairWidth: 0.3
• crosshairGap: 10
• Do not interpolate voxels
• etc

However when loading a niivue document, if these parameters aren't explicitly set in the document, they get overridden by niivue defaults.

For example, compare the difference in crosshair color and width when:

• loading a volume
• loading a niivue document

Come up with a system to preserve FreeBrowse defaults when loading a niivue document unless they are explicitly specified in the document.