pywrap: core: Add ToByteArray() context method.

This allows us to pass the context pixel data to various python
libraries (such as PySide).

The downside is that data are being coiped to newly allocated ByteArray.
I would love to see a native buffer implementation for GP_Context
however I doubt that we can easily fill out the PyTypeObject field
tp_as_buffer in swig without doing unspeakable things.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>