Thanks for the comments guys, hi racer-x.
Here is what I found.
Nvidia made graphics cards specifically for bitcoin miners that use the Linux-based 'ethOS'.

This card has no video connectors at all and is equivalent to a GTX 1060.
Mandelbulber can render on multiple graphics cards using OpenCL - the more GPUs the faster it renders.
Cryptocurrency mining must do something similar. Due to a crash in bitcoin, the cards are on ebay for hundreds of dollars less than equivalent GPUs.
I bought one for $74au to go with a GTX1060. Surprisingly it worked straight off but apparently only on Linux. Windows has 'signed driver enforcement' that stops them working. Of course, gamers wanted to use them as well.
I do all compositing with Gimp using python scripts written by rich2005, ofnuts and trandoductin. Three very talented people! I also use G'MIC and GAP. I found that Gimp is memory dependent so to anyone using it for video compositing, you need a lot of ram. Otherwise Gimp will start using the hard disk which really slows it down.