Made overlay_line use dpoints instead of point and also made it render the line

from the middle of the pixel.

dlib/gui_widgets/widgets.cpp

@@ -6375,8 +6375,8 @@ namespace dlib
         for (unsigned long i = 0; i < overlay_lines.size(); ++i)
         {
             draw_line(c, 
-                      zoom_in_scale*overlay_lines[i].p1/zoom_out_scale + origin, 
-                      zoom_in_scale*overlay_lines[i].p2/zoom_out_scale + origin, 
+                      zoom_in_scale*(overlay_lines[i].p1+dpoint(0.5,0.5))/zoom_out_scale + origin, 
+                      zoom_in_scale*(overlay_lines[i].p2+dpoint(0.5,0.5))/zoom_out_scale + origin, 
                       overlay_lines[i].color, area);
         }
 

dlib/gui_widgets/widgets.h

@@ -3337,11 +3337,11 @@ namespace dlib
             overlay_line() { assign_pixel(color, 0);}
 
             template <typename pixel_type>
-            overlay_line(const point& p1_, const point& p2_, pixel_type p) 
+            overlay_line(const dpoint& p1_, const dpoint& p2_, pixel_type p) 
                 : p1(p1_), p2(p2_) { assign_pixel(color, p); }
 
-            point p1;
-            point p2;
+            dpoint p1;
+            dpoint p2;
             rgb_alpha_pixel color;
         };
 

dlib/gui_widgets/widgets_abstract.h

@@ -2489,8 +2489,8 @@ namespace dlib
                     its two end points (p1 and p2) as well as a color.
             !*/
 
-            point p1;
-            point p2;
+            dpoint p1;
+            dpoint p2;
             rgb_alpha_pixel color;
 
             overlay_line(
@@ -2504,8 +2504,8 @@ namespace dlib
 
             template <typename pixel_type>
             overlay_line(
-                const point& p1_,
-                const point& p2_,
+                const dpoint& p1_,
+                const dpoint& p2_,
                 pixel_type p
             ); 
             /*!