Fix JoyCon triggers on the web

[Nintorch]

• I confirm that I am the author of this code and release it to the SDL project under the Zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

Description

This PR fixes JoyCon controller triggers detection in case they're in mini gamepad mode.
Previously they weren't detected because the browser reported having 2 real axes, while the SDL code expected to have at least 4.

The triggers still don't work correctly in Firefox, because they're detected as d-pad inputs, so I think that's a bug in the browser itself.

Existing Issue(s)

None

[slouken]

This PR assumes that any controller with two axes will have trigger buttons, but that's not necessarily the case. I haven't tried it, but I can imagine an SNES controller falling into this code path. Do you have any other way of positively identifying this case?

[Nintorch]

Hm, would it be acceptable to always have at least 6 axes and always read the trigger values from the buttons in case the mapping is standard, on both Chromium and Firefox? (and other browsers, of course)