Add preliminary support of Wayland under macOS

Wayland under macOS is WIP at
  https://github.com/owl-compositor/owl

src/drivers/Wayland/Fl_Wayland_Screen_Driver.H

@@ -169,7 +169,7 @@ public:
   char  *get_seat_name();
   struct xkb_keymap *get_xkb_keymap();
   static bool own_output(struct wl_output *output);
-  typedef enum {unspecified, MUTTER, WESTON, KDE} compositor_name;
+  typedef enum {unspecified, MUTTER, WESTON, KDE, OWL} compositor_name;
   static compositor_name compositor; // identifies the used Wayland compositor
   void set_spot(int font, int height, int x, int y, int w, int h, Fl_Window *win) FL_OVERRIDE;
   void reset_spot() FL_OVERRIDE;

src/drivers/Wayland/Fl_Wayland_Screen_Driver.cxx

@@ -1056,8 +1056,10 @@ static void registry_handle_global(void *user_data, struct wl_registry *wl_regis
   } else if (strcmp(interface, "org_kde_plasma_shell") == 0) {
     Fl_Wayland_Screen_Driver::compositor = Fl_Wayland_Screen_Driver::KDE;
     //fprintf(stderr, "Running the KDE compositor\n");
-  }
+  } else if (strncmp(interface, "zowl_mach_ipc", 13) == 0) {
-  else if (strcmp(interface, zwp_text_input_manager_v3_interface.name) == 0) {
+    Fl_Wayland_Screen_Driver::compositor = Fl_Wayland_Screen_Driver::OWL;
+    //fprintf(stderr, "Running the Owl compositor\n");
+  } else if (strcmp(interface, zwp_text_input_manager_v3_interface.name) == 0) {
     scr_driver->text_input_base = (struct zwp_text_input_manager_v3 *) wl_registry_bind(wl_registry, id, &zwp_text_input_manager_v3_interface, 1);
     //printf("scr_driver->text_input_base=%p version=%d\n",scr_driver->text_input_base,version);
   }

src/drivers/Wayland/Fl_Wayland_Window_Driver.cxx

@@ -873,6 +873,16 @@ static void xdg_toplevel_configure(void *data, struct xdg_toplevel *xdg_toplevel
   }
   window->configured_width = ceil(width / f);
   window->configured_height = ceil(height / f);
+  if (Fl_Wayland_Screen_Driver::compositor == Fl_Wayland_Screen_Driver::OWL) {
+    Fl_Window *sub = Fl::first_window();
+    while (sub) { // search still un-exposed sub-windows
+      if (sub->window() == window->fl_win) {
+        Fl_Window_Driver::driver(sub)->wait_for_expose_value = 0;
+        break;
+      }
+      sub = Fl::next_window(sub);
+    }
+  }
 }
 
 
@@ -1145,7 +1155,9 @@ void Fl_Wayland_Window_Driver::makeWindow()
   if (pWindow->menu_window() || pWindow->tooltip_window()) { // a menu window or tooltip
     is_floatingtitle = process_menu_or_tooltip(new_window);
 
-  } else if ( pWindow->border() && !pWindow->parent() ) { // a decorated window
+    // Don't attempt to use libdecor with OWL
+  } else if (Fl_Wayland_Screen_Driver::compositor != Fl_Wayland_Screen_Driver::OWL &&
+             pWindow->border() && !pWindow->parent() ) { // a decorated window
     new_window->kind = DECORATED;
     if (!scr_driver->libdecor_context)
       scr_driver->libdecor_context = libdecor_new(Fl_Wayland_Screen_Driver::wl_display, &libdecor_iface);
@@ -1175,7 +1187,11 @@ void Fl_Wayland_Window_Driver::makeWindow()
     // "A sub-surface becomes mapped, when a non-NULL wl_buffer is applied and the parent surface is mapped."
     new_window->configured_width = pWindow->w();
     new_window->configured_height = pWindow->h();
+    if (Fl_Wayland_Screen_Driver::compositor != Fl_Wayland_Screen_Driver::OWL) {
+      // With OWL, delay zeroing of subwindow's wait_for_expose_value until
+      // after their parent is configured, see xdg_toplevel_configure().
       wait_for_expose_value = 0;
+    }
     pWindow->border(0);
     checkSubwindowFrame(); // make sure subwindow doesn't leak outside parent