gl: do not include GL headers in public gstgl headers

Except for gst/gl/gstglfuncs.h

It is up to the client app to include these headers.
It is coherent with the fact that gstreamer-gl.pc does not
require any egl.pc/gles.pc. I.e. it is the responsability
of the app to search these headers within its build setup.

For example gstreamer-vaapi includes explicitly EGL/egl.h
and search for it in its configure.ac.

For example with this patch, if an app includes the headers
  gst/gl/egl/gstglcontext_egl.h
  gst/gl/egl/gstgldisplay_egl.h
  gst/gl/egl/gstglmemoryegl.h
it will *no longer* automatically include EGL/egl.h and GLES2/gl2.h.
Which is good because the app might want to use the gstgl api only
without the need to bother about gl headers.

Also added a test: cd tests/check && make libs/gstglheaders.check

https://bugzilla.gnome.org/show_bug.cgi?id=784779

ext/gtk/Makefile.am

@@ -27,7 +27,8 @@ libgstgtk_la_CFLAGS = \
 	$(GTK3_CFLAGS) \
 	$(GST_CFLAGS) \
 	$(GST_BASE_CFLAGS) \
-	$(GST_PLUGINS_BASE_CFLAGS)
+	$(GST_PLUGINS_BASE_CFLAGS) \
+	$(GL_CFLAGS)
 libgstgtk_la_LIBADD = \
 	$(GTK3_LIBS) \
 	$(GST_BASE_LIBS) \

ext/gtk/gstgtkglsink.c

@@ -27,6 +27,8 @@
 #include "config.h"
 #endif
 
+#include <gst/gl/gstglfuncs.h>
+
 #include "gstgtkglsink.h"
 #include "gtkgstglwidget.h"
 

ext/gtk/gtkgstglwidget.c

@@ -26,6 +26,7 @@
 
 #include "gtkgstglwidget.h"
 #include "gstgtkutils.h"
+#include <gst/gl/gstglfuncs.h>
 #include <gst/video/video.h>
 
 #if GST_GL_HAVE_WINDOW_X11 && GST_GL_HAVE_PLATFORM_GLX && defined (GDK_WINDOWING_X11)

tests/examples/gtk/glliveshader.c

@@ -20,6 +20,7 @@
 
 #include <gst/gst.h>
 #include <gst/gl/gl.h>
+#include <gst/gl/gstglfuncs.h>
 #include <gtk/gtk.h>
 #if GST_GL_HAVE_WINDOW_X11
 #include <X11/Xlib.h>