read-only buffers and memoryviews

[totaam]

When there can be no valid reason for modifying the buffer.

Implemented for some memoryviews in c623055

[totaam]

Many of the calls to PyMemoryView_FromMemory used the wrong call arguments (bool instead of enum ints) - presumably making the memoryview read-write when it should not be.

The only place where we want to use PyBUF_READ (read-only) memory is the XShmImageWrapper, since we don't own that memory. I don't think we ever write to that memory, and even if we did, I don't think this would be much of a problem.