ZX81 true interlaced hires graphics

Here is the source and binary to show the famous test-image Lena in 256x480 interlaced on a ZX81 with 16k WRX.
Code was done in ZX-IDE. Please excuse the poor source, it is a proof of concept, not a good coding example.

I developed this code about 10 years ago, and it is high time someone else saw it and could try it out.
This is the reason EightyOne has the option 'Interlace' in TV Emulation.
This should work on NTSC-equipment as well, but I have no true NTSC TVs to test it on.
It should be possible to combine with B0D0's 320-pixel resolution to get up to 320x512i, but that would require 24k RAM.
The interlace-vsyncs are not really following the standard. It would be possible to follow the standard with half-syncs if needed, but would require some (hairy) code.

Tested on:
- PAL ZX81 with 56k internal WRX-enabled RAM
- PAL ZX81, unmodified, with zxpand
- PAL B/W 5.5" TV, rf and composite video
- computer video-in card, rf and composite video
- Commodore 1081, composite video