Don't use shifted virtual modifier masks

Don't use shifted virtual modifier masks

Modifier masks can be confusing in some places. For example,
key->vmodmap only contains virtual modifiers, where the first is in
position 0, the second in 1 etc., while normally in a xkb_mod_mask_t the
virtual modifiers start from the 8th (XKB_NUM_CORE_MODS) position. This
happens in some other places as well.

Change all of the masks to be in the usual real+virtual format, and when
we need to access e.g. keymap->vmods we just adjust by
XKB_NUM_CORE_MODS. (This also goes for indexes, e.g.
interpret->virtual_modifier).

This makes this stuff easier to reason about.

Signed-off-by: Ran Benita <ran234@gmail.com>