Fix initial GetMouseDelta on GLFW

The initial mouse delta is often very large, depending on where the
cursor was before the window opened. This is because the first mouse
delta is the x/y coordinates your cursor entered the screen from. This
can (and often does) cause the free camera to make a sudden jump when
its moved for the first time.

The solution to this is to initialize the previousPosition and the
currentPosition to the same value on the first mouse input, essentially
ignoring the first mouse delta.

src/platforms/rcore_desktop_glfw.c

@@ -1869,10 +1869,18 @@ static void MouseButtonCallback(GLFWwindow *window, int button, int action, int
 // GLFW3 Cursor Position Callback, runs on mouse move
 static void MouseCursorPosCallback(GLFWwindow *window, double x, double y)
 {
+    static bool firstMouseInput = true;
+
     CORE.Input.Mouse.currentPosition.x = (float)x;
     CORE.Input.Mouse.currentPosition.y = (float)y;
     CORE.Input.Touch.position[0] = CORE.Input.Mouse.currentPosition;
 
+    if (firstMouseInput)
+    {
+        CORE.Input.Mouse.previousPosition = CORE.Input.Mouse.currentPosition;
+        firstMouseInput = false;
+    }
+
 #if defined(SUPPORT_GESTURES_SYSTEM) && defined(SUPPORT_MOUSE_GESTURES)
     // Process mouse events as touches to be able to use mouse-gestures
     GestureEvent gestureEvent = { 0 };