allow to rewind to frame 0

see #1371

- If we're at frame == 0, there's nowhere to rewind to, so the button is properly ignored.
- If we're at frame > 1, the logic is sending us to the previous frame, but it seems to be forcing emulation of that frame rather than just loading the state and stopping, and that is by design.
- If we're at frame 2, we want to arrive to frame 1, the logic will load state 0 and fastforward from that to 1. That way we don't have to store framebuffer in rewind states.
- If we're exactly at frame 1, we know we want to arrive to frame 0, but the logic implies emulating to frame 0, which is impossible. So instead it loads state 0 and emulates one frame as it's used to.

BizHawk.Client.EmuHawk/MainForm.cs

@@ -4467,7 +4467,7 @@ namespace BizHawk.Client.EmuHawk
 
 					if (isRewinding)
 					{
-						runFrame = true; // TODO: the master should be deciding this!
+						runFrame = Emulator.Frame > 1; // TODO: the master should be deciding this!
 						Master.Rewind();
 					}
 				}
@@ -4501,7 +4501,7 @@ namespace BizHawk.Client.EmuHawk
 
 				if (isRewinding)
 				{
-					runFrame = Global.Rewinder.Rewind(1);
+					runFrame = Global.Rewinder.Rewind(1) && Emulator.Frame > 1;
 
 					if (runFrame && Global.MovieSession.Movie.IsRecording)
 					{