add arc drawing and restructure draw functions

lv_draw/lv_draw_rect.h

@@ -27,6 +27,14 @@ extern "C" {
  * GLOBAL PROTOTYPES
  **********************/
 
+/**
+ * Draw a rectangle
+ * @param coords the coordinates of the rectangle
+ * @param mask the rectangle will be drawn only in this mask
+ * @param style pointer to a style
+ */
+void lv_draw_rect(const lv_area_t * coords, const lv_area_t * mask, const lv_style_t * style);
+
 /**********************
  *      MACROS
  **********************/