Move everything into “namespace movit”. This is a pretty hard API break, but it's probably the last big API break before 1.0, and some of the names (e.g. Effect, Input ResourcePool) are really so generic that they should not be allowed to pollute the global namespace.
Move to 'using namespace std;' in all .cpp files. There's no intrinsic value to writing std:: over and over again. We keep it in the .h file, of course, in order not to pollute clients' namespaces.
Rename .glsl to .vert/.frag.
Rename output_glsl to output_fragment_shader.
Move saturation out into its own effect, and hook up the control.