Print replay speed in FPS display

[zeamador]

Prints the replay playback rate (e.g. "1/2x", "2x", or "pause" if at the minimum rate) after the FPS in the FPS display during film playback.

Skips speed 0 so there aren't two settings that both play at 1x speed.

Also doubles the limits so we can go down to 1/10x or up to 10x.

[treellama]

One quirk I noticed: it won't display "paused" if the frame rate is set to 30 fps (because it never redraws the screen)

[treellama]

This will leak memory each time get_replay_speed_string() is called

[zeamador]

This will leak memory each time get_replay_speed_string() is called

D'oh, good point. I suppose it probably makes more sense to just move the formatting code directly into update_fps_display?

[treellama]

Normally you would either return a pointer to a static buffer (like strerror does), or pass the buffer and length in from the calling function (like snprintf). The former is not re-entrant, but we don't need re-entrancy here, so that's probably the way I'd fix it.