The main usage of ShaderManager::isValid was to have OpenGL2 specific
code pathes. Now we have an actual OpenGL2Compositing type and we know
that the ShaderManager is valid if we have this compositing type and we
know that it is not valid on OpenGL1Compositing. This gives us a much
better check and allows us to use the isValid method just for where we
want to check whether the shaders compiled successfully.

In addition some effects require OpenGL2, so we do not need to check
again that the ShaderManager is valid. Such usages are removed.