glslstage: delete shader on finalize of stage
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.