gdk: Codify the requirement that gsize is pointer-sized

This is widely assumed, but is not guaranteed by Standard C, and is
known to be false on CHERI architectures (which have 64-bit sizes and
128-bit tagged pointers). Add a static assertion to ensure that GTK
will not build on platforms where this assumption does not hold.

As discussed on GNOME/gtk!7510, if GTK switches from gsize to uintptr_t
as its representation of the underlying bits in a pointer, GTK maintainers
would prefer that to be done project-wide so that it's done consistently,
after which this static assertion could be removed.

At the time of writing, GLib makes the same assumption (GNOME/glib#2842),
but GLib contributors are gradually removing it (mostly by replacing gsize
with uintptr_t where a pointer-sized quantity is needed). Finishing
that work in GLib would be a prerequisite for being able to make GTK
work on the affected platforms.

Signed-off-by: Simon McVittie <smcv@debian.org>

gdk/gdk.c

@@ -43,6 +43,14 @@
 
 #include <fribidi.h>
 
+/* GTK has a general architectural assumption that gsize is pointer-sized
+ * (equivalent to uintptr_t), making it non-portable to architectures like
+ * CHERI where that isn't true. If a future release relaxes that
+ * assumption, changes will be needed in numerous places.
+ * See also https://gitlab.gnome.org/GNOME/glib/-/issues/2842 for the
+ * equivalent in GLib, which would be a prerequisite. */
+G_STATIC_ASSERT (sizeof (gsize) == sizeof (void *));
+G_STATIC_ASSERT (G_ALIGNOF (gsize) == G_ALIGNOF (void *));
 
 /**
  * GDK_WINDOWING_X11: