Fixed a problem with draw_line where it didn't always redraw the line properly.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402694

dlib/gui_widgets/canvas_drawing.h

@@ -107,15 +107,19 @@ namespace dlib
                     y = static_cast<long>(dy);
                     x = static_cast<long>(dx);
 
-                    if (y < valid_area.top() || y+1 > valid_area.bottom() )
-                        continue;
 
+                    if (y >= valid_area.top() && y <= valid_area.bottom())
+                    {
                         alpha_pixel.alpha = static_cast<unsigned char>((1.0-(dy-y))*max_alpha);
                         assign_pixel(c[y-c.top()][x-c.left()], alpha_pixel);
+                    }
+                    if (y+1 >= valid_area.top() && y+1 <= valid_area.bottom())
+                    {
                         alpha_pixel.alpha = static_cast<unsigned char>((dy-y)*max_alpha);
                         assign_pixel(c[y+1-c.top()][x-c.left()], alpha_pixel);
                     }
                 }         
+            }
             else
             {
                 const double slope = ((double)run)/rise;
@@ -145,16 +149,19 @@ namespace dlib
                     y = static_cast<long>(dy);
                     x = static_cast<long>(dx);
 
-                    if (x < valid_area.left() || x+1 > valid_area.right() )
-                        continue;
-
+                    if (x >= valid_area.left() && x <= valid_area.right())
+                    {
                         alpha_pixel.alpha = static_cast<unsigned char>((1.0-(dx-x))*max_alpha);
                         assign_pixel(c[y-c.top()][x-c.left()], alpha_pixel);
+                    }
+                    if (x+1 >= valid_area.left() && x+1 <= valid_area.right())
+                    {
                         alpha_pixel.alpha = static_cast<unsigned char>((dx-x)*max_alpha);
                         assign_pixel(c[y-c.top()][x+1-c.left()], alpha_pixel);
                     }
                 } 
             }
+        }
 
     }
     inline void draw_line (