video-overlay-composition: fix GSlice alloc/free size mismatch

video-overlay-composition: fix GSlice alloc/free size mismatch

Fix copy'n'paste bug which made us allocate a slice of the
size of a rectangle for the overlay composition, but then
free it passing the size of an overlay composition, which
is not something GSlice takes to kindly, resulting in scary
aborts like:

***MEMORY-ERROR***: GSlice: assertion failed: sinfo->n_allocated > 0

Also, g_slice_new already includes a cast, so remove our
own casts, without which the compiler would probably have
told us about this ages ago.

https://bugzilla.gnome.org/show_bug.cgi?id=680091