Fully test mesh functionality.

This was painful. The test currently fails for a few reasons, see
following commits.

src/Mesh.h

@@ -304,7 +304,6 @@ drawing commands are used on desktop OpenGL and OpenGL ES 3.0. See also
 
 @todo Support for indirect draw buffer (OpenGL 4.0, @extension{ARB,draw_indirect})
 @todo Redo in a way that allows glMultiDrawArrays, glDrawArraysInstanced etc.
-@todo test vertex specification & drawing
 @todo How to glDrawElementsBaseVertex()/vertex offset -- in draw()?
  */
 class MAGNUM_EXPORT Mesh: public AbstractObject {