liblbx: Implement new image reading API.

Add a new API which allows the application to essentially loop over
the drawing commands in an LBX frame.  This requires no runtime memory
allocations in the library, pushing all that out to the caller.

Conceptually this simplifies many applications because they can use
this to handle image data row-at-a-time.

For now, keep the old getframe function but change it to use the new
API under the hood.