state: support pure virtual modifiers

Traditionally, virtual modifiers were merely name aliases for real modifiers,
e.g. NumLock was usually mapped to Mod2 (see modifier_map statement). Virtual
modifiers that were never mapped to a real one had no effect on the keymap state.

This patch introduces the concept of pure virtual modifiers, i.e. virtual
modifiers that are not mapped now show up as if the were true modifiers.

Note that pure virtual modifiers cannot be used in an interpret action's AnyOf()
and an interpret action for a pure virtual modifier must be AnyOfOrNone() to
take effect:

    virtual_modifiers APureMod,...;

    interpret a+AnyOfOrNone(all) {
      virtualModifier= APureMod;
      action= SetMods(modifiers=APureMod);
    };

The above adds a pure virtual modifier for keysym `a`.

Interestingly, this fixes one current issue with our tests: previously the
de(neo) layout level5 didn't take effect correctly, with this patch in place it
now behaves.

Author: whot

src/xkbcomp/keymap.c

@@ -349,6 +349,24 @@ UpdateDerivedKeymapFields(struct xkb_keymap *keymap)
             if (key->vmodmap & (UINT32_C(1) << i))
                 mod->mapping |= key->modmap;
 
+    /*
+     * Virtual modifiers that are not mapped, map to themselves. This excludes:
+     * - Modifiers mapped *explicitly*: e.g. `virtual_modifiers VMod1 = 0x1;`.
+     * - Modifiers mapped *implicitly* via vmodmap/modifier_map, i.e. the usual
+     *   way used in xkeyboard-config.
+     */
+    xkb_vmods_enumerate(i, mod, &keymap->mods) {
+        const xkb_mod_mask_t mask = UINT32_C(1) << i;
+        if (mod->mapping == 0 && !(keymap->mods.explicit_vmods & mask)) {
+            mod->mapping = mask;
+            /*
+             * Make the mapping explicit, so that it is interoperable without
+             * relying on the implementation-specific virtual modifiers indices.
+             */
+            keymap->mods.explicit_vmods |= mask;
+        }
+    }
+
     /* Update the canonical modifiers state mask */
     assert(keymap->canonical_state_mask == MOD_REAL_MASK_ALL);
     xkb_mod_mask_t extra_canonical_mods = 0;