Add equality op that compares against a clamped integer
Fix window opacity not being applied on vert (re)construction
Remove Serializable's virtual destructor. It didn't do anything anyway.

To still make the compiler happy, add virtual destructors
to all descendants of Serializable.
Fix Tilemap's scanrow z sorting for good

Also make it a tad faster.
Make this mess of performance measuring a bit more readable
Make 'rb_get_args()' va_arg passing safer by introducing a termination marker

What can I say. I made a pact with the devil, and paid dearly.
Almost a whole day's worth of debugging, actually. Not again.

If this turns out to be slow we can always optimize the critical
parts (with no variable param count) later, or completely remove it.