add orbit camera support to FRED2 and qtFRED

[Goober5000]

Implement a spherical-coordinate orbit camera for both the MFC (FRED2) and Qt (qtFRED) editors, providing intuitive 3D viewport navigation similar to other 3D editing tools.

Controls:

• Middle mouse drag: orbit rotate around pivot point
• Shift + middle mouse drag: pan the pivot point
• Right mouse drag: orbit rotate (alternative to middle mouse)
• Shift + right mouse drag: pan (alternative to shift + middle mouse)
• Mouse wheel: zoom in/out
• Right click without drag: context menu (preserving existing feature)

Camera pivot selection:

• If an object is selected, orbit around that object
• Otherwise, intersect the camera forward ray with the current grid plane to find a natural pivot point
• Falls back to grid center if the camera is parallel to the grid

Grid-plane awareness:

• Orbit axes are derived from The_grid->gmatrix using vm_vec_rotate and vm_vec_unrotate, so the camera orbits correctly on all three grid planes (XZ, XY, YZ), not just the default XZ plane
• The look-at matrix uses the grid's up vector (uvec) rather than a hardcoded world-up direction

Robust zoom/pan scaling:

• Zoom uses an exponential formula (powf) so the distance multiplier is always positive regardless of physics_speed setting (1-500)
• Pan uses a clamped speed factor to avoid excessive movement at high physics_speed values

State management:

• Keyboard camera movement (process_controls) invalidates orbit state, so the next mouse drag re-initializes from the current view
• Orbit state is per-viewport in qtFRED (EditorViewport members) and global in FRED2 (matching existing conventions in each codebase)

Implementation:

• FRED2: orbit math in fredrender.cpp, mouse handlers in fredview.cpp
• qtFRED: orbit math in EditorViewport.cpp, mouse/wheel handlers in renderwidget.cpp; RenderWindow::event() updated to forward middle button and wheel events to RenderWidget