[gnome-games] gamepad-mapping-builder: Add a comment explaining X/Y swap

[Alexander Mikhaylenko <alexm src gnome org>]

commit 320e9c11bcc437e5fd0f638c23d05901911ad2ca
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Fri Feb 28 01:09:58 2020 +0500

    gamepad-mapping-builder: Add a comment explaining X/Y swap
    
    Input is messy, make it a bit less of a mess.

 src/gamepad/gamepad-mapping-builder.vala | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/src/gamepad/gamepad-mapping-builder.vala b/src/gamepad/gamepad-mapping-builder.vala
index 6533119a..2550f708 100644
--- a/src/gamepad/gamepad-mapping-builder.vala
+++ b/src/gamepad/gamepad-mapping-builder.vala
@@ -6,6 +6,14 @@ private class Games.GamepadMappingBuilder : Object {
                string source;
        }
 
+       // SDL strings are based on XBox button order unlike libretro, e.g.
+       // A is the bottom button rather than the right one.
+       //
+       // At the same time, /usr/include/linux/input-event-codes.h uses XBox order
+       // for A and B, and Nintendo order for X and Y: X is the top button, and Y
+       // is the left one.
+       //
+       // As a result, we swap X and Y again to match XBox order.
        private const GamepadInputSource[] INPUT_SOURCES = {
                { { EventCode.EV_ABS, EventCode.ABS_X }, "leftx" },
                { { EventCode.EV_ABS, EventCode.ABS_Y }, "lefty" },