Initialize screen gamma to "sane" defaults on OSX.

Can't find anything analogous for X11, unfortunately.
OSX & X11 infrastructure for pointer screen-edge detection.

Remote-mode only detects edge arrivals/departures by movements
requested by the master, which I think is probably desirable.
Move master info out of struct config and into its own struct.
Split handle_master_message() out of read_rmtdata().
Protocol support for EDGEMASKCHANGE messages.
Add get_current_modifiers() platform function.

OSX implementation is a stub.
Only trigger edge events when no mouse buttons are held.
Convert float flash duration to uint microseconds in parser.
Switch-by-mouse support.

Configurable number of taps within a configurable time window
switches in the directon of the tapped edge.