code shortcut, fix bugs in the addition and subtraction, another one in
the multiplication code, and add new unit tests for most of these.

to 32 bigits.

and add a test suite check for unary minus.

multiplication.

According to David Turner, “it should be the only thing that NDK users
should be testing again.”

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.