passing a buffer object to canvas.toBuffer to reduce gc thrash

[mreinstein]

toBuffer is very handy for blitting a canvas into some other graphics system:

const buffer = canvas.toBuffer('raw')
// blit buffer here

It would be really nice to pass in an existing buffer as an optional parameter, so it could be re-filled on each toBuffer invocation.
That would cut down on gc pressure quite a bit, especially in a tight render loop.