Add Model Viewer

[kzsh]

Model viewer changes:

• Adds GuiRotatingModelDebugView, a debug-only panel over the rotating
  model view with light azimuth and elevation sliders and a
  light-position marker.
• Adds a model preview to the GM object creation list and always shows
  the base model viewer on the GM screen; the model debug view is
  reachable from the home screen in debug builds.
• debug viewer model area is taller so the debug controls don't
  crowd the model.
• Mouse-wheel zoom is now multiplicative at 10% per notch instead of a
  flat 0.5 step.
• Included code for a normal-map toggle, but disabled until a pending PR with normal maps lands.

Notes:

• If there are performance or clutter concerns with rendering a model on the GM screen, we could add an options.ini toggle to enable/disable it, or remove it entirely.
• The render pipeline for the GuiRotatingModelDebugView performs a lot of rendering logic parts of which match other rendering pipelines in the code. Might be nice to de-duplicate it at some point, but because of the conditional rendering of various shaders, and the experimental nature of the debug model viewer, it felt ok to keep it as separate code. I'm open to alterations.

Screen caps

start screen

Model debug view

Gm Screen