Revert "mesa: remove FLUSH_VERTICES() in _mesa_MatrixMode()"

This reverts commit 0de5e0f3fb.

Michel Dänzer spotted two piglit regressions from the change.  I suspect
that removing the FLUSH_VERTICES() actually exposed a bug elsewhere but
I don't have time to hunt down the root issue at this time.

src/mesa/main/matrix.c

@@ -151,6 +151,7 @@ _mesa_MatrixMode( GLenum mode )
 
    if (ctx->Transform.MatrixMode == mode && mode != GL_TEXTURE)
       return;
+   FLUSH_VERTICES(ctx, _NEW_TRANSFORM);
 
    switch (mode) {
    case GL_MODELVIEW: