webkitgtk: add an option to control USE_GSTREAMER_GL

webkitgtk: add an option to control USE_GSTREAMER_GL

This covers the case where GL/GLES is available (so -DENABLE_OPENGL=ON
gets passed), which makes the webkitgtk build system assume GStreamer-GL
is available, while actually it is not.

Also, providing an option to manually disable usage of GStremer-GL can
help with certain target configurations in which using OpenGL for video
handling might result in incorrect rendering.

This fixes some autobuilder failures like the following:

  http://autobuild.buildroot.net/results/187796535af53ece426641ff7d88aabada281674
  http://autobuild.buildroot.net/results/00c1a8ea23a99728a4f3f4478705f2383414ae41

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>