Do binding modifier lookup on XKB state, not physical keys

When we update the modifier_state used for Weston bindings, derive this
from the XKB modifier state, rather than a hardcoded mapping of physical
keys to modifier state.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>
diff --git a/src/compositor.h b/src/compositor.h
index 26857c2..4c037c9 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -305,6 +305,9 @@
 		struct xkb_rule_names names;
 		struct xkb_context *context;
 		struct xkb_keymap *keymap;
+		xkb_mod_index_t ctrl_mod;
+		xkb_mod_index_t alt_mod;
+		xkb_mod_index_t super_mod;
 	} xkb_info;
 };