Support viewport feature for the scene.

[hermet]

Since setting up a viewport region is a common method, we can officially support it as a replacement concept:

As is:

//use a shape for clipping
auto shape = Shape::gen();
shape->appendRect(x, y, w, h);
...
//setup its viewport
scene->composite(tvg::CompositeMethod::Clip, move(shape));

To be:

//No need to create additional data.
scene->viewport(x, y, w, h);

This is a more optimal approach for our rendering pipeline.

• A viewport requires only four coordinates, whereas a shape has numerous properties allocated.
• When the scene includes a composition, a viewport cannot be applied with a clipper; an additional scene layer is needed.