Any pad can control the menu

[gblues]

Description

== DETAILS
I am not sure I've quite got it so that any pad can open the
menu, but I do have it so any pad can control it.

• split out the input processing into a separate method
• track down and squish some hairy bugs that boiled down to
  bad pointer math
• verified that this change did not impact actual cores

Reviewers

@twinaphex

[jdgleaver]

@gblues Thanks - this is an often requested feature!

There are a few issues, though:

• This must be optional - i.e. users must be able to enforce menu control by player 1 only
• In collect_system_input() you loop over MAX_USERS ports. This is needlessly inefficient - you only need to loop over all enabled ports
• In collect_system_input(), the keyboard and AI service handling should not go inside the loop. Keyboard input is always read from port 0, and the AI code already loops over MAX_USERS
• The merge_input_bits() function is... just wrong...

I have prepared a small patch that fixes these issues:
gblues-menu-input-v2.patch.zip
(EDIT: I just noticed a minor inefficiency in the original patch I uploaded - the v2 patch addresses this)

Please apply this on-top of your PR and check that everything still works :)

Also, feel free to set DEFAULT_ALL_USERS_CONTROL_MENU to true for your target platform (it's the WiiU, right?)

Final note: With this PR, the toggle menu gamepad hotkey will only work for player 1, because hotkeys are handled differently and it would need a rewrite... But any set Menu Toggle Controller Combo will work fine for all users, so I think that's acceptable.