GLSLstage creates the glShader using glCreateShader, but never calls
glDeleteShader if the glShader is not used anymore. This forces the GL
library to keep the compiled shader around, because it might be used in
the future. Therefore, the glShader is leaked whenever a GLSLStage is
destroyed.

Fix the leak by deleting the glShader when finishing the GLSLStage.

Part-of: <gstreamer/gst-plugins-base!886>