Fix relative mouse data being really inconsistent in SDL

src/platforms/rcore_desktop_sdl.c

@@ -1025,7 +1025,7 @@ void PollInputEvents(void)
     CORE.Input.Mouse.currentWheelMove.y = 0;
 
     // Register previous mouse position
-    if (platform.cursorRelative) CORE.Input.Mouse.currentPosition = (Vector2){ 0.0f, 0.0f };
+    if (platform.cursorRelative) CORE.Input.Mouse.previousPosition = (Vector2){ 0.0f, 0.0f };
     else CORE.Input.Mouse.previousPosition = CORE.Input.Mouse.currentPosition;
 
     // Reset last gamepad button/axis registered state