Improve the performance of the ImplicitPlaneRepresentation

In VR the ImplicitPlaneRepresentation was very slow mainly
due to the plane widget being 128x128 cells and then fed into
FeatureEdges every time the plane was modified (which is every
frame when interacting). So this has the following changes

1. Change the plane resolution to 1x1 which seems to work fine and
   significanly speeds up the pipeline when manipulating the plane.

2. Remove the Line*2 ivars and instead just make the first line go the
   full length instead of being broken into two. This reduces the number
   of sources, mappers and actors all of which slow down updates.

3. Update the representation rebuild code to not rebuild the geometry
   when the renderwindow changes. None of the calculations dependended
   on the renderwindow except the handle resize. So moved that outside
   the if statement.

4. Remove a reinterpret_cast that wasn't required from
   ImplicitPlaneWidget2.