More PLS simplifications

1) Remove the "FIRST_VERTEX_OF_CONTOUR" flag. Rather than relying on
   this flag, the vertex shader can know it's crossed into the wrong
   contour when the tessellation vertex's contourID does not match what
   we expect it to be.

2) PLSRenderer shouldn't have to think about
   PLSRenderContext::m_tessVertexCount. Remove the getter for this value
   and move all the logic and asserts dealing with it into
   PLSRenderContext.

Diffs=
7b3d53e53 More PLS simplifications (#5626)

Co-authored-by: Chris Dalton <99840794+csmartdalton@users.noreply.github.com>