computed on demand with static initialisation.

on Windows if using Visual Studio; if using MinGW, try to stick to the
original main() entry point.

do not open a console at startup.

Remez exchange program.

and the OS X port.

to ios-image.cpp, but disable it on OS X because UIKit isn't available.

Implement GetMouseCount() to query the number of mice on the system.

tested wherever relevant.

sub-millisecond but can be improved if we get rid of SDL timers.