More work on heightmap probing.

docs/todo.txt

@@ -46,6 +46,12 @@ Corner finding
     - need to know the bit diameter
     - use G38.2 to probe X edge and set that as current X - diam/2
 
+Render arcs
+Render quadratic splines
+Render bezier splines
+Synchronize executing program with render (show executed paths with different colors)
+Show a progress bar when executing programs.
+Add aggressive buffer execution policy and make it selectable.
 
 DONE - Show program extents