Feature: Standardize panning orientation with MMB between spring and overhead cams

Overhead performs the 'natural' panning (camera moves opposite of mouse delta direction, as if grabbing the ground), while spring camera does the opposite.

Moving between camera modes makes this orientation to invert. Make this behavior consistent, pick an orientation and stick with it. Users and games can set their preference with `InvertMouse` or by providing positive/negative speeds.

Start by locating the overhead and spring camera implementations for middle-mouse panning, then trace how InvertMouse and positive or negative speeds are applied. Compare the two camera modes' mouse-delta directions and verify that switching modes preserves one consistent orientation while retaining the documented preference controls.

Written by the indexing model from the issue text.