Removed warnings printed when GL functions are not found.
 - Missing functions may well be the normal state; all OpenGL stacks are not equal.
 - If we attempt to _use_ a missing function, a descriptive error is raised,
   so it is unlikely anyone will miss these messages.
Merge branch 'master' into visual_transform_chains
Previously, record arrays with only a single vield were interpreted as
having no fields at all. This has the awkward consequence that
vbo['field_name'] does not work when the dtype has only one field.
minor edits
fixed line visual example
added PolarTransform