Added ctx->Vertex/FragmentProgram._Enable flags. Set when vertex/fragment

program is enabled AND the currently bound program is valid.
Check _Enable instead of Enable to prevent things from blowing up
when someone calls glEnable(GL_VERTEX_PROGRAM_ARB) without actually
defining a program.

src/mesa/tnl/t_vb_program.c

@@ -315,7 +315,7 @@ static GLboolean run_init_vp( GLcontext *ctx,
  */
 static void check_vp( GLcontext *ctx, struct tnl_pipeline_stage *stage )
 {
-   stage->active = ctx->VertexProgram.Enabled;
+   stage->active = ctx->VertexProgram._Enabled;
 
    if (stage->active) {
       /* Set stage->inputs equal to the bitmask of vertex attributes