Option to stream image

[joeserg]

Hi there,

I was wondering if we could have the option to stream the image to the browser, rather than saving it to disk?

In my application I would like to show a thumbnail of a dynamically generated PDF, so the image is only relevant for that particular request.

[freekmurze]

I'd accept a PR that adds a getImageData-function to the package. The function should contain some of the code currently present in saveImage. saveImage should use the new getImageData-function.