710: Implement read_buffer r=kvark a=fintelia
I'm not sure about the exact interface that makes sense, but this PR sketches out how read_buffer (and eventually also read_texture) could be implemented.
Resolves#694
Co-authored-by: Dzmitry Malyshau <kvarkus@gmail.com>
Co-authored-by: Jonathan Behrens <fintelia@gmail.com>