Initial support for relative mouse mode and skeleton of support for capture mode. Avoid mouse position overflow in absolute mode. Allow absolute mode to be set by ROM DB.

--- a/rom.db	Mon Dec 14 19:36:01 2015 -0800
+++ b/rom.db	Tue Dec 15 20:01:50 2015 -0800
@@ -375,4 +375,5 @@
 		2 mouse.1
 		ext none
 	}
+	mouse_mode absolute
 }