Missing features from RTI viewers like OpenLime

[jo-chemla]

Libs and viewers like cnr-isti-vclab/OpenLime implements a batch of useful features for paintings/frescoes viewing, RTI style, that could be ported to terrain-viewer - eg in a non-geo mode:

• light control in the viewport (pressing shortcut key)
• lens
• gallery like OSD
• custom shaders
• terradraw (annotations equivalent)
• lightsphere miniviewer
• minimap

etc