Add API for buffer age about softbuffer HOT 1 CLOSED

The way this interacts with buffer resizing is interesting. Depending on the backend, it may be able to grow or shrink a buffer while using the same memory. If height increased, it may only be necessary to draw the extra height. If width changed, everything needs to be shifted over.

It could return an age of None here like a fresh buffer, but we'll have to be clear that this doesn't guarantee a zeroed buffer then (not that it matters, generally, since an application typically won't rely on the default value of the buffer).

from softbuffer.