One may not have an GstGLContext available or current in the thread where one
would need to update the shader. Support this by signalling create-shader
whenever the one-shot 'update-shader' is set to TRUE.
Implemented with videotestsrc ! glshader ! glupload ! gtkglsink
Errors on an invalid shader compilation are ignored however any error
provided by the glsl compiler is printed to stdout.
