Feature request: stride parameter in push methods.

[erikcorry]

At the moment the push routines take just 'w' which is used both as the width of the image and it is multiplied by 4 to give the distance in bytes between the adjacent lines in the input.
If you separate out those two things then I can push a small rectangle from a buffer into a PNG.

The legacy/simple call push(buf, x, y, w, h) can be implemented as pushRectangle(buf, x, y, w*4, w, h)