Fix fl_read_image() under hybrid Wayland/X11 platform.

Function fl_read_image() obliges to keep a minimal use of global variable fl_window
also under the Wayland platform, even if its type (Window) makes little sense for
the hybrid library because it has its X11 value (given by X11/X.h) which is not
meaningful for the Wayland leg of the hybrid platform.

Virtual member function Fl_Surface_Device::as_image_surface() becomes useless.

FL/Fl_Device.H

@@ -92,7 +92,6 @@ public:
   }
   /** \brief The destructor. */
   virtual ~Fl_Surface_Device();
-  virtual Fl_Image_Surface *as_image_surface();
   static void push_current(Fl_Surface_Device *new_current);
   static Fl_Surface_Device *pop_current();
 };

FL/Fl_Image_Surface.H

@@ -82,7 +82,6 @@ public:
   int printable_rect(int *w, int *h);
   Fl_Offscreen offscreen();
   void rescale();
-  Fl_Image_Surface *as_image_surface();
 };
 
 
@@ -113,7 +112,6 @@ protected:
   virtual void untranslate() = 0;
   int printable_rect(int *w, int *h);
   virtual Fl_RGB_Image *image() = 0;
-  virtual Fl_Image_Surface *as_image_surface();
   /** Each platform implements this function its own way.
    It returns an object implementing all virtual functions
    of class Fl_Image_Surface_Driver for the plaform.

FL/platform.H

@@ -39,9 +39,6 @@ class Fl_Window;
 #    include "mac.H"
 #  elif defined(FLTK_USE_WAYLAND)
 #    include "wayland.H"
-#     if !FLTK_USE_X11
-        typedef struct wld_window *Window;
-#     endif
 #  elif defined(FLTK_USE_X11)
 #    include "x11.H"
 #  endif // _WIN32

FL/wayland.H

@@ -45,6 +45,8 @@ extern FL_EXPORT EGLContext fl_wl_glcontext(GLContext rc);
 #  if FLTK_USE_X11
 //  *********** for X11 component ***********
 #    include "x11.H"
+#  else
+      typedef struct wld_window *Window;
 #  endif
 
 #endif // FL_DOXYGEN

src/CMakeLists.txt

@@ -667,7 +667,7 @@ if (OPTION_USE_WAYLAND)
   list (APPEND OPTIONAL_LIBS "-lwayland-cursor -lwayland-client -lxkbcommon -ldl -ldbus-1")
   if (NOT OPTION_WAYLAND_ONLY)
     list (APPEND OPTIONAL_LIBS "-lXcursor -lXrender -lXinerama -lXfixes -lXft -lXext -lX11")
-  endif (OPTION_WAYLAND_ONLY)
+  endif (NOT OPTION_WAYLAND_ONLY)
   if (GTK_FOUND)
     list (APPEND OPTIONAL_LIBS ${GTK_LDFLAGS} )
   endif (GTK_FOUND)

src/Fl_Device.cxx

@@ -97,11 +97,6 @@ Fl_Surface_Device::~Fl_Surface_Device()
   if (surface_ == this) surface_ = NULL;
 }
 
-/** Returns non-NULL if this surface is an Fl_Image_Surface object
- \version 1.4.0
- */
-Fl_Image_Surface *Fl_Surface_Device::as_image_surface() { return NULL; }
-
 /**  A constructor that sets the graphics driver used by the display */
 Fl_Display_Device::Fl_Display_Device(Fl_Graphics_Driver *graphics_driver) : Fl_Surface_Device(graphics_driver) {
   this->set_current();

src/Fl_Image_Surface.cxx

@@ -90,9 +90,6 @@ int Fl_Image_Surface_Driver::printable_rect(int *w, int *h) {
   return 0;
 }
 
-Fl_Image_Surface *Fl_Image_Surface_Driver::as_image_surface() {
-  return image_surface_;
-}
 /**
  \}
  \endcond
@@ -155,11 +152,6 @@ void Fl_Image_Surface::rescale() {
 }
 
 
-Fl_Image_Surface *Fl_Image_Surface::as_image_surface() {
-  return this;
-}
-
-
 // implementation of the fl_XXX_offscreen() functions
 
 static Fl_Image_Surface **offscreen_api_surface = NULL;

src/drivers/Wayland/Fl_Wayland_Image_Surface_Driver.cxx

@@ -60,12 +60,14 @@ void Fl_Wayland_Image_Surface_Driver::set_current() {
   ((Fl_Wayland_Graphics_Driver*)fl_graphics_driver)->set_buffer((struct fl_wld_buffer*)offscreen);
   pre_window = Fl_Wayland_Window_Driver::wld_window;
   Fl_Wayland_Window_Driver::wld_window = NULL;
+  fl_window = 0;
 }
 
 void Fl_Wayland_Image_Surface_Driver::end_current() {
   cairo_surface_t *surf = cairo_get_target(((struct fl_wld_buffer *)offscreen)->cairo_);
   cairo_surface_flush(surf);
   Fl_Wayland_Window_Driver::wld_window = pre_window;
+  fl_window = (Window)pre_window;
 }
 
 void Fl_Wayland_Image_Surface_Driver::translate(int x, int y) {

src/drivers/Wayland/Fl_Wayland_Window_Driver.cxx

@@ -51,7 +51,7 @@ extern "C" {
 #define fl_min(a,b) ((a) < (b) ? (a) : (b))
 
 #if !FLTK_USE_X11
-Window fl_window = 0; // temporary, until we get rid of this global for wayland
+Window fl_window = 0;
 #endif
 
 
@@ -362,6 +362,7 @@ void Fl_Wayland_Window_Driver::make_current() {
   }
 
   Fl_Wayland_Window_Driver::wld_window = window;
+  fl_window = (Window)window;
   float scale = Fl::screen_scale(pWindow->screen_num()) * window->scale;
   if (!window->buffer) {
     window->buffer = Fl_Wayland_Graphics_Driver::create_shm_buffer(

src/fl_read_image.cxx

@@ -40,9 +40,12 @@
 uchar *fl_read_image(uchar *p, int X, int Y, int w, int h, int alpha) {
   uchar *image_data = NULL;
   Fl_RGB_Image *img;
-  // TODO: strive to remove use of the fl_window global variable in platform-independent code
-  //if (Fl_Surface_Device::surface()->as_image_surface()) { // read from off_screen buffer
-  if (fl_find(fl_window)==0) { // read from off_screen buffer
+  // Under macOS and Wayland, fl_window == 0 when an Fl_Image_Surface is the current drawing
+  // surface. Otherwise, fl_window corresponds to a mapped Fl_Window.
+  // Under X11 and windows, fl_window is an offscreen buffer when an Fl_Image_Surface
+  // is the current drawing or when fl_read_image() is called inside the draw() of
+  // an Fl_Double_Window.
+  if (fl_find(fl_window) == 0) { // read from offscreen buffer or buffer of an Fl_Double_Window
     img = Fl::screen_driver()->read_win_rectangle(X, Y, w, h, 0);
     if (!img) {
       return NULL;