Fix GCMouse scroll axis mapping on macOS

[hollinsky]

The scroll valueChangedHandler was treating xValue as vertical and yValue as horizontal, copied from the iOS GCMouse implementation. On macOS, the GCControllerDirectionPad axes follow their documented semantics: xAxis (left/right) is horizontal and yAxis (up/down) is vertical.

Swap the axis assignment so xValue maps to horizontal scroll and yValue maps to vertical scroll.

Tested on macOS 26.3.1, using both the internal trackpad of my MacBook Pro M1 Pro and a wireless USB mouse, and ultimately fixes scrolling for me in Dear ImGui.

[whrvt]

Something to note: this doesn't respect the "natural scrolling" option in system settings, which is kind of unfortunate. I'm not familiar with these macOS APIs or what to do in this situation myself.

Edit: actually, it seems like there are multiple issues:

• The Y axis is incorrectly inverted here, causing inverted scrolling in general (so I believe it should just be yValue instead of -yValue here)
• The initial system "Natural Scrolling" setting isn't taken into account unless it's changed while the application is already running. Separate issue unrelated to this PR and the GCMouse implementation, but caused my initial confusion about the setting not being taken into account at all.