Support Wayland only (without X11 support in gdk) (#21218)

Adds a support for compiling flutter engine when
gdk does not have X11 backend. In such a configuration
the generated gdkconfig.h header file looks like the following:

 /* gdkconfig.h
  *
  * This is a generated file.  Please modify `configure.ac'
  */

 #ifndef __GDKCONFIG_H__
 #define __GDKCONFIG_H__

 #if !defined (__GDK_H_INSIDE__) && !defined (GDK_COMPILATION)
 #error "Only <gdk/gdk.h> can be included directly."
 #endif

 #include <glib.h>

 G_BEGIN_DECLS

 #define GDK_WINDOWING_WAYLAND

 G_END_DECLS

 #endif  /* __GDKCONFIG_H__ */

Additionally headers like <gdk/gdkx.h> are not available at all.

Above configuration can be found on the most of the embedded systems.

This patch enables compilation of X11 specific code only when gdk
defines GDK_WINDOWING_X11.
Signed-off-by: NDamian Wrobel <dwrobel@ertelnet.rybnik.pl>

shell/platform/linux/fl_renderer_x11.cc

@@ -3,6 +3,8 @@
 // found in the LICENSE file.
 
 #include "fl_renderer_x11.h"
+#ifdef GDK_WINDOWING_X11
+
 #include "flutter/shell/platform/linux/egl_utils.h"
 
 struct _FlRendererX11 {
@@ -108,3 +110,5 @@ static void fl_renderer_x11_init(FlRendererX11* self) {}
 FlRendererX11* fl_renderer_x11_new() {
   return FL_RENDERER_X11(g_object_new(fl_renderer_x11_get_type(), nullptr));
 }
+
+#endif  // GDK_WINDOWING_X11

shell/platform/linux/fl_renderer_x11.h

@@ -5,6 +5,9 @@
 #ifndef FLUTTER_SHELL_PLATFORM_LINUX_FL_RENDERER_X11_H_
 #define FLUTTER_SHELL_PLATFORM_LINUX_FL_RENDERER_X11_H_
 
+#include <gdk/gdk.h>
+
+#ifdef GDK_WINDOWING_X11
 #include <gdk/gdkx.h>
 
 #include "flutter/shell/platform/linux/fl_renderer.h"
@@ -35,4 +38,6 @@ FlRendererX11* fl_renderer_x11_new();
 
 G_END_DECLS
 
+#endif  // GDK_WINDOWING_X11
+
 #endif  // FLUTTER_SHELL_PLATFORM_LINUX_FL_RENDERER_X11_H_

shell/platform/linux/fl_view.cc

@@ -5,7 +5,9 @@
 #include "flutter/shell/platform/linux/public/flutter_linux/fl_view.h"
 
 #include <gdk/gdkwayland.h>
+#ifdef GDK_WINDOWING_X11
 #include <gdk/gdkx.h>
+#endif
 #include <cstring>
 
 #include "flutter/shell/platform/linux/fl_engine_private.h"
@@ -131,17 +133,27 @@ static void fl_view_plugin_registry_iface_init(
   iface->get_registrar_for_plugin = fl_view_get_registrar_for_plugin;
 }
 
+static FlRenderer* fl_view_get_renderer_for_display(GdkDisplay* display) {
+#ifdef GDK_WINDOWING_X11
+  if (GDK_IS_X11_DISPLAY(display)) {
+    return FL_RENDERER(fl_renderer_x11_new());
+  }
+#endif
+
+  if (GDK_IS_WAYLAND_DISPLAY(display)) {
+    return FL_RENDERER(fl_renderer_wayland_new());
+  }
+
+  g_error("Unsupported GDK backend");
+
+  return nullptr;
+}
+
 static void fl_view_constructed(GObject* object) {
   FlView* self = FL_VIEW(object);
 
   GdkDisplay* display = gtk_widget_get_display(GTK_WIDGET(self));
-  if (GDK_IS_X11_DISPLAY(display)) {
-    self->renderer = FL_RENDERER(fl_renderer_x11_new());
-  } else if (GDK_IS_WAYLAND_DISPLAY(display)) {
-    self->renderer = FL_RENDERER(fl_renderer_wayland_new());
-  } else {
-    g_error("Unsupported GDK backend");
-  }
+  self->renderer = fl_view_get_renderer_for_display(display);
   self->engine = fl_engine_new(self->project, self->renderer);
 
   // Create system channel handlers.