Windows platform: use GDI+ to antialias oblique lines and curves.

FL/Fl_Graphics_Driver.H

@@ -374,6 +374,8 @@ public:
   virtual float override_scale();
   virtual void restore_scale(float);
   virtual PangoFontDescription* pango_font_description(Fl_Font fnum) { return NULL; }
+  virtual void antialias(int state);
+  virtual int antialias();
 };
 
 #ifndef FL_DOXYGEN

FL/fl_draw.H

@@ -238,6 +238,16 @@ enum {
   FL_JOIN_BEVEL = 0x3000        ///< join style: line join is tidied
 };
 
+/**
+ Turns ON or OFF antialiased line drawings, if supported by platform.
+ Currently, only the Windows platform allows to change whether line drawings are antialiased.
+ Turning it OFF may accelerate heavy drawing operations.
+ */
+inline void fl_antialias(int state) { fl_graphics_driver->antialias(state); }
+
+/** Returns whether line drawings are currently antialiased */
+inline int fl_antialias() { return fl_graphics_driver->antialias(); }
+
 // rectangles tweaked to exactly fill the pixel rectangle:
 
 /**