vtkContext2D takes a new type of item which directly draws a 2D mesh
represented by a vtkPolyData object. Internally, vtkContextDevice2D makes
the API calls necessary to draw the internal primitives of vtkPolyData.
Added a test demonstrating the use of vtkPolyDataItem.

Added interaction and batched primitives for drawing.

vtkInteractiveArea derives from vtkContextArea and implements basic interaction
(zoom and panning). Currently, two types of vtkPolyData primitives are supported,
Lines and Polygons. Lines are batched and drawn using ContextDevice2D::DrawLines.
Polygons are first decomposed into a set of triangles and then drawn using
ContextDevice2D::CoreDrawTriangles. Primitives are colored per vertex.

Suppressed run-time warning.

Setting the current renderer in the interactor style. Removed
unecessary initializations.

Using defined symbolic names for scalar modes.