Add aligned allocator functions to GSK

Add aligned allocator functions to GSK

The code is mostly stolen from graphene.
Allocators support any alignment, but their implementation
only calls system aligned allocator functions if malloc()
is not aligned to 16-byte boundaries. If it is aligned,
the implementation just calls malloc() regardless of which
alignment is requested by the caller.

This can be fixed by saving the result of meson malloc()
alignment check and adding a few conditions to the implementation,
but right now GSK and GTK only need 16-byte alignment either way.