Remove obsolete/disabled "boxcheat" code

Windows platform: Remove comments about obsolete code and one
exported declaration of the variable 'fl_background_pixel' which
is used only on the X11 platform.

X11 platform: Remove comments.

Other platforms: not affected.

FL/win32.H

@@ -1,7 +1,7 @@
 //
-// Windows system header file for the Fast Light Tool Kit (FLTK).
+// Windows platform header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2018 by Bill Spitzak and others.
+// Copyright 1998-2022 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -41,7 +41,6 @@ typedef HWND Window;
 
 extern FL_EXPORT UINT fl_wake_msg;
 extern FL_EXPORT char fl_override_redirect; // hack into Fl_Window::make_xid()
-extern FL_EXPORT int fl_background_pixel;  // hack into Fl_Window::make_xid()
 extern FL_EXPORT HPALETTE fl_palette; // non-zero only on 8-bit displays!
 extern void fl_release_dc(HWND w, HDC dc);
 extern FL_EXPORT void fl_save_dc( HWND w, HDC dc);

src/Fl_win32.cxx

@@ -1977,7 +1977,6 @@ private:
 
 void fl_fix_focus(); // in Fl.cxx
 
-// int fl_background_pixel = -1; // color to use for background
 UINT fl_wake_msg = 0;
 int fl_disable_transient_for; // secret method of removing TRANSIENT_FOR
 
@@ -2612,20 +2611,8 @@ int Fl_WinAPI_Window_Driver::set_cursor(const Fl_RGB_Image *image, int hotx, int
 ////////////////////////////////////////////////////////////////
 // Implement the virtual functions for the base Fl_Window class:
 
-// If the box is a filled rectangle, we can make the redisplay *look*
-// faster by using X's background pixel erasing.  We can make it
-// actually *be* faster by drawing the frame only, this is done by
-// setting fl_boxcheat, which is seen by code in fl_drawbox.cxx:
-// For Windows it looks like all windows share a background color, so
-// I use FL_GRAY for this and only do this cheat for windows that are
-// that color.
-// Actually it is totally disabled.
-// Fl_Widget *fl_boxcheat;
-// static inline int can_boxcheat(uchar b) {return (b == 1 || (b & 2) && b <= 15);}
-
 void Fl_WinAPI_Window_Driver::show() {
   if (!shown()) {
-    // if (can_boxcheat(box())) fl_background_pixel = fl_xpixel(color());
     makeWindow();
   } else {
     // Once again, we would lose the capture if we activated the window.

src/Fl_x.cxx

@@ -3198,16 +3198,9 @@ void Fl_X11_Window_Driver::label(const char *name, const char *iname) {
 ////////////////////////////////////////////////////////////////
 // Implement the virtual functions for the base Fl_Window class:
 
-// If the box is a filled rectangle, we can make the redisplay *look*
+// If the box is a filled rectangle, we can make the redisplay
-// faster by using X's background pixel erasing.  We can make it
+// *look* faster by using X's background pixel erasing.
-// actually *be* faster by drawing the frame only, this is done by
+
-// setting fl_boxcheat, which is seen by code in fl_drawbox.cxx:
-//
-// On XFree86 (and prehaps all X's) this has a problem if the window
-// is resized while a save-behind window is atop it.  The previous
-// contents are restored to the area, but this assumes the area
-// is cleared to background color.  So this is disabled in this version.
-// Fl_Window *fl_boxcheat;
 static inline int can_boxcheat(uchar b) {return (b==1 || ((b&2) && b<=15));}
 
 void Fl_X11_Window_Driver::show() {

src/fl_boxtype.cxx

@@ -438,7 +438,6 @@ void fl_draw_box(Fl_Boxtype t, int x, int y, int w, int h, Fl_Color c) {
   if (t && fl_box_table[t].f) fl_box_table[t].f(x,y,w,h,c);
 }
 
-//extern Fl_Widget *fl_boxcheat; // hack set by Fl_Window.cxx
 /** Draws the widget box according its box style */
 void Fl_Widget::draw_box() const {
   if (box_) draw_box((Fl_Boxtype)box_, x_, y_, w_, h_, color_);