add arc drawing and restructure draw functions

lv_draw/lv_draw_line.h

@@ -26,6 +26,16 @@ extern "C" {
  * GLOBAL PROTOTYPES
  **********************/
 
+/**
+ * Draw a line
+ * @param p1 first point of the line
+ * @param p2 second point of the line
+ * @param maskthe line will be drawn only on this area
+ * @param lines_p pointer to a line style
+ */
+void lv_draw_line(const lv_point_t * point1, const lv_point_t * point2, const lv_area_t * mask,
+                  const lv_style_t * style);
+
 /**********************
  *      MACROS
  **********************/