Bugfix: DataChangeListener must always store the maximum modification flag inside webglData.

Problem:
1. Value Size Changed
2. Value Value Changed
3. Rendering / Buffer Update
During the buffer update, the renderer assumes that only the value changed.
Thus, only the value gets updated to a buffer of incorrect size.
Xflow bugfixes.
Xflow change: Empty buffers are now always returned as 'null'.

Thus, for the result there is no difference between no buffer defined and empty buffer.
Added error message for attemp of render mesh without any vertex attributes (due to empty buffers).