gl: document GST_GL_* environment variables

https://bugzilla.gnome.org/show_bug.cgi?id=733245

ext/gl/gstglimagesink.c

@@ -30,6 +30,9 @@
  * If no Window ID was provided by the application, the element will
  * create its own internal window and render into it.
  *
+ * See the #GstGLDisplay documentation for a list of environment variables that
+ * can override window/platform detection.
+ *
  * <refsect2>
  * <title>Scaling</title>
  * <para>

gst-libs/gst/gl/gstgldisplay.c

@@ -31,6 +31,15 @@
  * Elements are required to make use of #GstContext to share and propogate
  * a #GstGLDisplay.
  *
+ * There are a number of environment variables that influence the choice of
+ * platform and window system specific functionality.
+ * - GST_GL_WINDOW influences the window system to use.  Common values are
+ *   'x11', 'wayland', 'win32' or 'cocoa'.
+ * - GST_GL_PLATFORM influences the OpenGL platform to use.  Common values are
+ *   'egl', 'glx', 'wgl' or 'cgl'.
+ * - GST_GL_API influences the the OpenGL API requested by the OpenGL platform.
+ *   Common values are 'opengl' and 'gles2'.
+ *
  * <note>Certain window systems require a special function to be called to
  * initialize threading support.  As this GStreamer GL library does not preclude
  * concurrent access to the windowing system, it is strongly advised that