apprt/gtk: force X11 backend on GTK 4.14

I'm unsure if this is an environmental issue just for me or if this is
more widespread or what other downsides this may have. I'm more than
willing to revert this if it ends up causing different issues.

I found that with NixOS 24.11 and GTK 4.14 on my system, the default
Wayland GDK backend fails to initialize EGL. With GTK 4.16 everything
is fine.

If I force X11 then everything also works fine. This commit forces X11
for GTK 4.14 specifically (4.16+ is allowed to use Wayland).

src/apprt/gtk/App.zig

@@ -115,8 +115,23 @@ pub fn init(core_app: *CoreApp, opts: Options) !App {
         // reassess...
         //
         // Upstream issue: https://gitlab.gnome.org/GNOME/gtk/-/issues/6589
+        //
+        // Specific details about values:
+        //   - "opengl" - output OpenGL debug information
+        //   - "gl-disable-gles" - disable GLES, Ghostty can't use GLES
+        //   - "vulkan-disable" - disable Vulkan, Ghostty can't use Vulkan
+        //     and initializing a Vulkan context was causing a longer delay
+        //     on some systems.
         _ = internal_os.setenv("GDK_DEBUG", "opengl,gl-disable-gles,vulkan-disable");
+
+        // Wayland-EGL on GTK 4.14 causes "Failed to create EGL context" errors.
+        // This can be fixed by forcing the backend to X11. This issue appears
+        // to be fixed in GTK 4.16 but I wasn't able to bisect why.
+        _ = internal_os.setenv("GDK_BACKEND", "x11");
     } else {
+        // Versions prior to 4.14 are a bit of an unknown for Ghostty. It
+        // is an environment that isn't tested well and we don't have a
+        // good understanding of what we may need to do.
         _ = internal_os.setenv("GDK_DEBUG", "vulkan-disable");
     }