Change coordinate system for bottom view

[JayFoxRox]

I think we should unmirror the bottom image, so that it's easier to read labels and easier to compare to a physical board.

Anything affecting multiple board layers would have to be done in another file which does also contain a mirrored variant (which should be mirrored in the SVG, if possible, and not in the image file) or we'd just create a convention like "if a via is visible on top and bottom, it should be added to top_via.svg, it won't be added to bottom_via.svg" or something.

In https://github.com/LoveMHz/xbox-board-re/pull/7#discussion_r439342796, I've mentioned that we should probably split into separate files for different views of the board, and then automate the step of merging them together. That's when we could be flipping the coordinate system (either in our own script, or by using some SVG library or external tool).