All fixed:

testgtk.c: In function `create_gridded_geometry':
testgtk.c:1983: warning: dereferencing type-punned pointer will break
strict-aliasing rules
testgtk.c: In function `create_key_lookup':
testgtk.c:4159: warning: dereferencing type-punned pointer will break
strict-aliasing rules
testgtk.c: In function `find_widget_at_pointer':
testgtk.c:11253: warning: dereferencing type-punned pointer will break
strict-aliasing rules

Not fixed (not sure whether to remove these functions or not...):

testgtk.c: At top level:
testgtk.c:7304: warning: `opacity_toggled_cb' defined but not used
testgtk.c:7315: warning: `palette_toggled_cb' defined but not used