v0.2.6 — Controller fixes (prerelease)

This is a prerelease: it fixes controllers not working (#8) but has not been tried on Windows with a controller yet. If yours works now, or still doesn't, please say so in #8.

Downloads: the same three setup programs; use these instead of the earlier
ones and run the setup again to rebuild the game.

Fixed

• Controllers not working (#8). Only the first controller found was used;
  on Windows that is often a virtual or idle device (from Steam, DS4Windows,
  vJoy, a racing wheel...), and then nothing pressed on any controller worked.
  Every connected controller now works at once.
• Generic USB pads that SDL does not know the layout of were ignored. They
  are now read with the usual layout: the first buttons as A, B, X, Y, then L,
  R, Select, Start, with the D-pad and the left stick. A device whose stick
  rests at one end (some keyboards and mice report themselves as joysticks) has
  its stick ignored, so the player does not walk on their own.
• The controller now also works while the game window is not focused, as when
  it was started from the setup window.

Tested

• On Linux with a real Xbox Series controller and virtual ones (an Xbox 360
  pad, a generic USB pad, one with a stuck stick). Compiled for Windows, not yet
  tried with a controller there.