Summary

• Increase the maximum number of supported joystick axes to 16; instances of magic numbers for maximum number of axes/buttons/POV hats have been replaced with macros
• Raw input (Windows) backend: Add support for more axis types, including DirectInput slider axes; separate slider handling left over from the old backend has been removed
• SDL (Linux/Mac) backend: Fix axes beyond 6 being non-functional, properly enforce the maximum axis/button/POV limit, and fix pointless iterations over not-present axes and buttons
• Merge the joystick_*.h headers into gameport.h; this mirrors the treatment of other headers containing only device declarations

Checklist

• Closes CH Flightstick Pro emulation not working right #3825
• I have discussed this with core contributors already
• This pull request requires changes to the ROM set

References

N/A