[gtk/ebassi/docs-for-main] Document side effect of GLArea:has-depth-buffer

[Emmanuele Bassi <ebassi src gnome org>]

commit 8a6dd2805ff77113db5d8d64fd2c10c36cf67607
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Fri Jan 7 18:30:54 2022 +0000

    Document side effect of GLArea:has-depth-buffer
    
    We enable depth testing when creating a depth buffer.
    
    Fixes: #838

 gtk/gtkglarea.c | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/gtk/gtkglarea.c b/gtk/gtkglarea.c
index de7cc7fa0f..af4d46ec91 100644
--- a/gtk/gtkglarea.c
+++ b/gtk/gtkglarea.c
@@ -833,6 +833,10 @@ gtk_gl_area_class_init (GtkGLAreaClass *klass)
    *
    * If set to %TRUE the widget will allocate and enable a depth buffer for the
    * target framebuffer.
+   *
+   * Setting this property will enable GL's depth testing as a side effect. If
+   * you don't need depth testing, you should call `glDisable(GL_DEPTH_TEST)`
+   * in your `GtkGLArea::render` handler.
    */
   obj_props[PROP_HAS_DEPTH_BUFFER] =
     g_param_spec_boolean ("has-depth-buffer",