Strip modifier keys of modifiers.

For some reason modifiers are applied to modifier keys being held down when a mode is loaded.
I suspect this is a problem with xcb -- I haven't been able to find another culprit yet.
This strips modifiers of modifier keys. I don't see this breaking anything since modifiers
with modifier keys isn't normally applied and is nonesense from the beginning.

This partially fixes #5288. It doesn't fully fix the issue since B_UPON_KEYRELEASE_IGNORE_MODS
can't be applied to a key release binding that didn't get a keypress, so the order that keys are
released when the bindsym is made between modes matters when it shouldn't.

I'm not sure what exactly the fix for that would be-- and it seems like it'd be a much bigger change
than this, and that it would have to deserve it's own commit.

src/bindings.c

@@ -316,6 +316,25 @@ Binding *get_binding_from_xcb_event(xcb_generic_event_t *event) {
     /* Remove the CapsLock bit */
     i3_event_state_mask_t state_filtered = event_state & ~XCB_MOD_MASK_LOCK;
     DLOG("(removed capslock, state = 0x%x)\n", state_filtered);
+    /* Strip modifier keys of modifiers. Needed for keyrelease events between modes */
+    struct xkb_state *dummy_state = xkb_state_new(xkb_keymap);
+    xkb_keysym_t event_sym = xkb_state_key_get_one_sym(dummy_state,event_detail);
+    xkb_state_unref(dummy_state);
+    switch(event_sym) {
+        case XKB_KEY_Meta_L:
+        case XKB_KEY_Meta_R:
+        case XKB_KEY_Alt_L:
+        case XKB_KEY_Alt_R:
+        case XKB_KEY_Super_L:
+        case XKB_KEY_Super_R:
+        case XKB_KEY_Control_L:
+        case XKB_KEY_Control_R:
+        case XKB_KEY_Hyper_R:
+        case XKB_KEY_Hyper_L:
+            state_filtered = state_filtered & 0xFF00;
+            break;
+    }
+
     /* Transform the keyboard_group from bit 13 and bit 14 into an
      * i3_xkb_group_mask_t, so that get_binding() can just bitwise AND the
      * configured bindings against |state_filtered|.