Continuous camera zoom

#2810 changed camera zoom to zoom to the point under the mouse cursor.

If the cursor is toward the bottom of the screen, this can still require several right click and drags before zooming all the way in. The zoom should continue as long as the right button is down even if the mouse is not moving, almost like the inertia for rotation.

We may want to make this an option (default `true`?), but let's experiment with it first.

Start by tracing the camera-zoom change introduced in #2810 and the existing inertia-for-rotation behavior. Experiment with keeping zoom active while the right button remains down without mouse movement, and verify that zooming continues toward the cursor position as described.

Written by the indexing model from the issue text.