Basic Target module to manage render targets

My logic was: "What?" then "Where?" then "How?".
So ```CallClear()``` draw ```ClearData``` into the ```Frame```, without anything in "How?".
But ```CallDraw()``` renders a ```MeshHandle``` into the ```Frame```, and ```ProgramHandle``` with (to be defined) ```Environment``` definiting "How?".
If you want, we can totally change this scheme.