mesa/dlist: shortcircuit some redundant statechanges at compile time

mesa/dlist: shortcircuit some redundant statechanges at compile time

Currently, state-changes in mesa display lists are more or less
a verbatim recording of the GL calls made during compilation.

This change introduces a minor optimization to recognize and eliminate
cases where the application emits redundant state changes, eg:

  glShadeModel( GL_FLAT );
  glBegin( prim )
  ...
  glEnd()
  glShadeModel( GL_FLAT );
  glBegin( prim )
  ...
  glEnd()

The big win is when we can eliminate all the statechanges between two
primitive blocks and combine them into a single VBO node.

This commit implements state-change elimination for Material and ShadeModel
only.  This is enough to make a start on debugging, etc.