Allow mods to force-show the crosshair on touchscreen for bows etc.

[grorp]

On touchscreens, there is no crosshair by default. However, mods might still use the player's look direction for certain items, for example bows or guns. For these items, a crosshair should be shown.

This PR allows mods to force-show the crosshair by setting the HUD flag crosshair_force_show = true.

Alternative to the proposal in https://codeberg.org/mineclonia/mineclonia/pulls/890#issuecomment-2340468, cc @j-r

To do

This PR is a Ready for Review.

Feel free to comment on API design as well

How to test

1. Set touch_controls = true (also possible on desktop) and touch_use_crosshair = false (the default)
2. Start a Devtest world with Worldedit
3. The crosshair is hidden
4. //lua player:hud_set_flags({crosshair = true, crosshair_force_show = true}) -> The crosshair is shown
5. //lua player:hud_set_flags({crosshair = false, crosshair_force_show = true}) -> The crosshair is hidden

[grorp]

With this PR, Mineclonia can do:

mcl_player.register_globalstep(function(player)
    local name = player:get_wielded_item():get_name()
    local is_bow = name:sub(1, 12) == "mcl_bows:bow" or name:sub(1, 17) == "mcl_bows:crossbow"
    player:hud_set_flags({crosshair_force_show = is_bow})
end)

[SmallJoker]

It needs to be true by default, like all other HUD flags

Why? Client::handleCommand_HudSetFlag only modifies the touched flags, and older servers will not touch those (see ObjectRef::l_hud_set_flags -> no lookup possible). Hence crosshair_force_show = false by default should be possible.

[grorp]

Hence crosshair_force_show = false by default should be possible.

If it's possible, then it's definitely a much cleaner API. Updated, thank you.

[grorp]

Something that should also be considered: What happens to this flag if the crosshair becomes a builtin HUD element? See #14273. The server doesn't have access to the client's settings, so we'd still have to implement this flag in a "hardcoded" manner on the client (or send the setting to the server).

#14273 already needed to do that for the existing client-side conditions though:

But still needs a draw_crosshairs condition since CAMERA_MODE_THIRD_FRONT and HAVE_TOUCHSCREENGUI may disallow crosshairs.

So this is not really a new problem.