allow negative zoom-sensitivity to reverse scroll-zoom direction

[eyaler]

negative sensitivity is already supported for orbit angles. adding this will allow the enlarging zoom direction to be the same as the scroll down direction, allowing the user to scroll down to a model and continuing to scroll down to zoom in.

edit: this actually already works for me. so maybe it only needs a documentation remark (assuming there no edge cases depending on the zoom to be positive). another issue is that currently this also reverses the pinch on mobile which is not intended