input: check for ‘effective’ modifiers, not ‘depressed’

When reading XKB modifier state, use XKB_STATE_MODS_EFFECTIVE, not
XKB_STATE_MODS_DEPRESSED.

This fixes handling of ‘sticky’ modifier keys.

CHANGELOG.md

@@ -38,6 +38,12 @@
 ### Deprecated
 ### Removed
 ### Fixed
+
+* ‘Sticky’ modifiers in input handling; when determining modifier
+  state, foot was looking at **depressed** modifiers, not
+  **effective** modifiers, like it should.
+
+
 ### Security
 ### Contributors