Pacify GCC in pgtkselect malloc alignment

Pacify GCC in pgtkselect malloc alignment

* src/pgtkselect.c (pgtk_size_for_format): Remove.
(pgtk_get_window_property): Third arg is now void **, not unsigned
char **.  Omit last arg.  All callers changed.  Simplify
allocation to pacify GCC 14.2.1 complaint about malloc size
mismatching size of array: there’s no need for an extra byte when
converting from ldata to idata.
(selection_data_to_lisp_data): Third arg is now void const *,
not unsigned char const *.  All callers changed.
Omit unnecessary casts.  Make some locals more local.

(cherry picked from commit ff65cc9944dc0b37986d512ee8b9817c6913db36)