Camera tracking: support of tripod motion solving

Expose option into interface to use modal solver which currently
supports only tripod motion.

This solver requires two tracks at least to reconstruct motion.
Using more tracks aren't improving solution in general, just adds
instability into solution and slows down things a lot.

Refirement of camera intrinsics is supported by this solver.

To use this solver just activate "Tripod Motion" checkbox in
solver panel.

release/scripts/startup/bl_ui/space_clip.py

@@ -227,7 +227,11 @@ class CLIP_PT_tools_solve(Panel):
                      else "Object Motion")
         col.operator("clip.clear_solution")
 
+        col = layout.column()
+        col.prop(settings, "use_tripod_solver")
+
         col = layout.column(align=True)
+        col.active = not settings.use_tripod_solver
         col.prop(settings, "keyframe_a")
         col.prop(settings, "keyframe_b")