Improve handling of High-DPI mice

- Reduce lower bound of mouse sensitivity
 - Add error accumulators to prevent small motions from being rounded off

src/win/win_mouse.c

@@ -29,6 +29,7 @@
 
 int    mouse_capture;
 double mouse_sensitivity = 1.0; /* Unused. */
+double mouse_x_error = 0.0, mouse_y_error = 0.0; /* Unused. */
 
 typedef struct {
     int buttons;