glimagesink: fix asynchrone drawing

Simplify gst_glimage_sink_redisplay which is there
only to ask the window for a redraw.

Put a lock to make sure we are not realeasing
the stored buffer while still drawing the corresponding
texture
glimagesink: avoid to draw texture while releasing stored buffer

Spoted by generic/cube example with ClientDrawCallback set to
returning TRUE (it means drawing as fast as possible)
context: Reimplement GL context sharing

https://bugzilla.gnome.org/show_bug.cgi?id=704806
add GstGLContext test that tests context sharing
raspberrypi: add support for dispmanx

See https://bugzilla.gnome.org/show_bug.cgi?id=703342
rpi: call bcm_host_init in plugin_init
configure.ac: add -lGLU only if glu is present

It fixes build on rpi because for some reason libgl1-mesa
has to be installed even if useless
glfilter: rename external-opengl-context prop to other-context

It now requires a GstGLContext instead of a handle
egl: doesn't require the EGLConfig from the other context

libEGL should give us the same or similar EGLConfig from the
EGLDisplay we have.
context: Move some window error enums to context
x11: remove dead code
tests: update for GstGLContext addition
move the GL vtable from GstGLDisplay to GstGLContext
unref the pool on shutdown
mixer: timestamp buffers
fix some refcount errors