core: added GetMouseDelta() (#1832)

* core: added `GetMouseDelta()`

Thanks to previousPosition added by raysan it is now possible to create the GetMouseDelta() function.

Returns a Vector2 with the difference between the current and previous position of the mouse in a frame.

Useful for creating camera scrolling, among others.

* Added changes noted by raysan

src/core.c

@@ -3537,6 +3537,17 @@ Vector2 GetMousePosition(void)
     return position;
 }
 
+// Get mouse delta between frames
+Vector2 GetMouseDelta(void)
+{
+    Vector2 delta = {0};
+
+    delta.x = CORE.Input.Mouse.currentPosition.x - CORE.Input.Mouse.previousPosition.x;
+    delta.y = CORE.Input.Mouse.currentPosition.y - CORE.Input.Mouse.previousPosition.y;
+
+    return delta;
+}
+
 // Set mouse position XY
 void SetMousePosition(int x, int y)
 {

src/raylib.h

@@ -1113,6 +1113,7 @@ RLAPI bool IsMouseButtonUp(int button);                       // Check if a mous
 RLAPI int GetMouseX(void);                                    // Get mouse position X
 RLAPI int GetMouseY(void);                                    // Get mouse position Y
 RLAPI Vector2 GetMousePosition(void);                         // Get mouse position XY
+RLAPI Vector2 GetMouseDelta(void);                            // Get mouse delta between frames
 RLAPI void SetMousePosition(int x, int y);                    // Set mouse position XY
 RLAPI void SetMouseOffset(int offsetX, int offsetY);          // Set mouse offset
 RLAPI void SetMouseScale(float scaleX, float scaleY);         // Set mouse scaling