[gtk/wip.win32.fixes] GDK-Win32: Make EGL a runtime opt-in

[Chun-wei Fan <fanchunwei src gnome org>]

commit ef10a9a26b03cfbbbabca1837047383629e5c5ca
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Mon Nov 8 15:37:08 2021 +0800

    GDK-Win32: Make EGL a runtime opt-in
    
    Use the debug envvar 'GDK_DEBUG=gl-egl' to determine whether we want to try to
    initialize EGL first before trying WGL, as a means for people to more easily
    enable EGL support on Windows to test EGL there (such as to debug the shaders,
    for instance)

 gdk/win32/gdkdisplay-win32.c | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)
---
diff --git a/gdk/win32/gdkdisplay-win32.c b/gdk/win32/gdkdisplay-win32.c
index dd40b5c13a..d4a275ec7f 100644
--- a/gdk/win32/gdkdisplay-win32.c
+++ b/gdk/win32/gdkdisplay-win32.c
@@ -1152,21 +1152,23 @@ gdk_win32_display_init_gl_backend (GdkDisplay  *display,
   if (display_win32->dummy_context_wgl.hdc == NULL)
     display_win32->dummy_context_wgl.hdc = GetDC (display_win32->hwnd);
 
-  /* No env vars set, do the regular GL initialization, first WGL and then EGL,
+  /*
+   * No env vars set, do the regular GL initialization, first WGL and then EGL,
    * as WGL is the more tried-and-tested configuration.
    */
 
-/*
- * Disable defaulting to EGL for now, since shaders need to be fixed for
- * usage against libANGLE EGL.  EGL is used more as a compatibility layer
- * on Windows rather than being a native citizen on Windows
- */
-#if 0
-  result = gdk_display_init_egl (display,
-                                 EGL_PLATFORM_ANGLE_ANGLE,
-                                 display_win32->dummy_context_wgl.hdc,
-                                 FALSE,
-                                 error);
+#ifdef HAVE_EGL
+  /*
+   * Disable defaulting to EGL for now, since shaders need to be fixed for
+   * usage against libANGLE EGL.  EGL is used more as a compatibility layer
+   * on Windows rather than being a native citizen on Windows
+   */
+  if (_gdk_debug_flags & GDK_DEBUG_GL_EGL)
+    result = gdk_display_init_egl (display,
+                                   EGL_PLATFORM_ANGLE_ANGLE,
+                                   display_win32->dummy_context_wgl.hdc,
+                                   FALSE,
+                                   error);
 #endif
 
   if (!result)