allow to specify the framebuffer from the outside.

[simon-budig]

This pull request makes it possible to hand in a pointer to the framebuffer memory, so that it does not get allocated by the library itself.

This makes it possible for the application, where in the memory the framebuffer is located exactly.

Also this change makes the "pixels" member public.

To the best of my knowledge this is a backward-compatible change. I use it in https://github.com/simon-budig/flame/blob/master/flame.ino