gl: do not include GL headers in public gstgl headers

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