SDL does not refresh with workspace swaping

[sjrct]

When swapping away from an back to a running instance of the game, the window does not redraw until something happens.

[Grissess]

Suggested rectification: trigger either redraw() or just sdl_refresh() on a window expose event in sdl_default_handler().

This is presently defined in input.c, since most event handling is input handling, but that's also kind of silly because it means that display.c has to extern a lot of its state so that the default event handler can modify it.