External Mining GPU Mount

I really liked what b1sh0p did with his external video card mount. My remix changes a few minor things:

1. Smooth out the extended support arm.
2. Made the bracket hole go all the way to the bottom, so if you have sparse infill, you don't need to worry about printing over nothing.
3. Decrease width of the bracket hole (so the bottom of the video card when installed, doesn't have as much play)
4. Change bracket hole from block to rounded edge to match the arm design.
5. Lengthen the video card mount (where you screw the video card into the brace) to fit more snug.
6. Broke out the video card mount into its own STL so you can print it independent, in case you lose one or need to reprint it for whatever reason.

Video_card_mount.stl contains multiple holes, but I found only one was needed, plus the hole was a bit hard to thread into, so I made Video_card_mount_single.stl, which has a single hole, and its stepped at the entrance, so should be easier to thread into.

Optional: I found that with heavy cards, the GPU mount can sag. I wanted something underneath the riser to help keep it stable. I include two sizes of riser supports (20mm and 22mm, which one you need will depend on your video card and riser dimensions). Measure from the top of the GPU mount base to the bottom of your riser card to see how tall it needs to be. I think most people will be able to use the 20mm. It should snap into place without needing glue, though you can glue it in if you wanted. The ridge side will face down, and should fit snugly into the inside of the gpu stand.