memory_decompress not working when decompressing to VRAM

[bsb-RickD]

Decompressing to VRAM doesn't work at the moment. The reason for that is that the decompressor not only outputs bytes, but also needs to read already decompressed bytes when they are repeated in the output stream.
This is what LZSA calls a backreference.

For that to work, the second VERA dataport needs to be set up properly to read from earlier output bytes.

I have no idea if decompressing to other 9fxx addresses ever can work

[bsb-RickD]

I have created a pull request with a fix:
#371

[mooinglemur]

Pulled into the X16Community fork via X16Community/x16-rom#4