This adds --input-touch-emulate-mouse option, which controls whether to
enable legacy touch handling where touch inputs are emulated as mouse
inputs. This establishes a primary touch point (the one with the lowest
index) as the emulated mouse position, and MBTN_LEFT up/down with the
appearance of the first touch point and the disappearance of the last
touch point.
This fixes some problems with touch handling on Wayland, for example
attempting to pinch results in a double click.
