Fixes  for _enqueue_*_buffer_rect

Fixes for pyopencl.cffi_cl._enqueue_read_buffer_rect and pyopencl.cffi_cl._enqueue_write_buffer_rect, minor change to pyopencl.cffi_cl._enqueue_copy_buffer_rect to tuple-ise the pitch arguments.

Unit tests (test/test_enqueue_copy.py) for _enqueue_read_buffer_rect, _enqueue_write_buffer_rect and _enqueue_copy_buffer_rect.

Remove stray underscore from create_from_gl_texture