Feature request: Add vdp_read()

[plattysoft]

There is vdp_write and vdp_memcpy which are equivalents to VPOKE, but I am missing an equivalent function to VPEEK, which I would expect to be vdp_read.
Relying on the information of the VDP is a common pattern, so it would be good to have it.
I understand that in most cases you would have a buffer that you use memcpy to put on the vdp and you can read from it, but there are other cases when having a vdp_read is useful, such as low level animations (mutable tiles)

[retrodeluxe]

@plattysoft pull requests are welcome in this project, specially for small additions like this.

[plattysoft]

I don't know enough assembly or the internals of the engine to be able to write that part, otherwise I would have gone for it straight away.