Fixes to enqueue_copy_buffer_rect

There are two bugs:
1. If src_pitches or dst_pitches is specified, there is an uninitialised
   variable.
2. If ConstBuffer is given a short array and content==0, the internal
   buffer is not zero-filled, causing garbage to be passed on.

The latter probably affects a lot of other functions, but hopefully in a
positive way.

Fixes #105.