Fix picking bug:

On some GL implementations (mesa software rendering, some AMD cards on
Linux), picking would stop working when switching between pick targets.

The previous logic only updates a buffer when deemed necessary, however
in any case it is first invalidated via glBufferData.

I suppose nvidia graphics, where this worked, did not clear the buffer
and with some luck we got an unchanged buffer.

Fix: only use glBufferData if we actually need to change the buffer.