xvimagsink: fix failure to allocate large shared memory blocks

xvimagsink: fix failure to allocate large shared memory blocks

A previous patch increased allocations by 15 bytes in order to ensure
16 byte alignment for g_malloc blocks. However, shared memory is
already block aligned, and this extra 15 bytes caused allocation
to fail when we were already allocating to the shared memory limit,
which is a lot smaller than typical available RAM.

Fix this by removing the alignment slack when allocating shared
memory.

https://bugzilla.gnome.org/show_bug.cgi?id=706066