Fix "Mac: Most fl_symbols missing corner pixels" (#1409).

+ Fix line loop drawing on macOS (#1409)
+ macOS: yet another fix of line drawing (#1409)

All fixes backported and merged from master, commits:

 - ea1b1bddd9
 - e19bc59ec1
 - 3c8bc27458

src/drivers/Quartz/Fl_Quartz_Graphics_Driver.H

@@ -108,7 +108,9 @@ protected:
   int not_clipped(int x, int y, int w, int h) FL_OVERRIDE;
   void restore_clip() FL_OVERRIDE;
   void end_points() FL_OVERRIDE;
+  void end_loop() FL_OVERRIDE;
   void end_line() FL_OVERRIDE;
+  void end_line_or_loop(bool is_loop);
   void end_polygon() FL_OVERRIDE;
   void end_complex_polygon() FL_OVERRIDE;
   void circle(double x, double y, double r) FL_OVERRIDE;

src/drivers/Quartz/Fl_Quartz_Graphics_Driver_vertex.cxx

@@ -33,7 +33,16 @@ void Fl_Quartz_Graphics_Driver::end_points() {
   }
 }
 
+void Fl_Quartz_Graphics_Driver::end_loop() {
+  fixloop();
+  end_line_or_loop(true);
+}
+
 void Fl_Quartz_Graphics_Driver::end_line() {
+  end_line_or_loop(false);
+}
+
+void Fl_Quartz_Graphics_Driver::end_line_or_loop(bool is_loop) {
   if (n < 2) {
     end_points();
     return;
@@ -43,6 +52,8 @@ void Fl_Quartz_Graphics_Driver::end_line() {
   CGContextMoveToPoint(gc_, xpoint[0].x, xpoint[0].y);
   for (int i=1; i<n; i++)
     CGContextAddLineToPoint(gc_, xpoint[i].x, xpoint[i].y);
+  if (is_loop)
+    CGContextClosePath(gc_);
   CGContextStrokePath(gc_);
   CGContextSetShouldAntialias(gc_, false);
 }