Add fl_remove_scale()/fl_restore_scale() to transiently draw without scaling factor.

This new API is a response to this message in fltk.general :
Can custom box type functions handle their own high-DPI screen scaling?

FL/Fl_Device.H

@@ -81,7 +81,9 @@ public:
   /** \brief Returns the graphics driver of this drawing surface. */
   inline Fl_Graphics_Driver *driver() {return pGraphicsDriver; };
   /** The current drawing surface.
-   In other words, the Fl_Surface_Device object that currently receives all graphics requests */
+   In other words, the Fl_Surface_Device object that currently receives all graphics requests
+   \note It's possible to transiently remove the GUI scaling factor in force in the current
+   drawing surface with \ref fl_remove_scale(). */
   static inline Fl_Surface_Device *surface() {
     return surface_ ? surface_ : default_surface();
   };

FL/Fl_Graphics_Driver.H

@@ -364,6 +364,8 @@ public:
   virtual void font_name(int num, const char *name);
   // Defaut implementation may be enough
   virtual void overlay_rect(int x, int y, int w , int h);
+  virtual float remove_scale();
+  virtual void restore_scale(float);
 };
 
 #ifndef FL_DOXYGEN
@@ -486,6 +488,8 @@ protected:
   void transformed_vertex(double xf, double yf);
   virtual void transformed_vertex0(float x, float y);
   void vertex(double x, double y);
+  virtual float remove_scale();
+  virtual void restore_scale(float);
 };
 #endif // FL_DOXYGEN
 

FL/fl_draw.H

@@ -687,6 +687,11 @@ FL_EXPORT const char *fl_local_to_mac_roman(const char *t, int n=-1);
 
 /** \addtogroup  fl_drawings
     @{ */
+
+FL_EXPORT float fl_remove_scale();
+
+FL_EXPORT void fl_restore_scale(float s);
+
 /**
   Draws a nul-terminated UTF-8 string starting at the given \p x, \p y location.