mesa: Fix ff fragment shader inputs calculation when enabling a VS.

The FF VS generation happens just after the FF FS generation in
state.c, so the ctx->VP._Current value is for the previous state
update's vertex shader, not the one that will be chosen as a result of
this state update.  The vertexShader and vertexProgram variables
should be accurately telling us whether there's going to be a
ctx->VP._Current (except on _MaintainTnlProgram drivers, where it's
always true).

The glsl-vs-statechange-1 test was created to test for this, but it
turns out that the bug is hidden by the fact that we call
_mesa_update_state() twice per draw call -- once from
_mesa_valid_to_render() and once from vbo_draw_arrays(), and the
second one was fixing up the first one.

Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/main/ff_fragment_shader.cpp

@@ -330,8 +330,7 @@ static GLbitfield get_fp_input_mask( struct gl_context *ctx )
       /* _NEW_RENDERMODE */
       fp_inputs = (FRAG_BIT_COL0 | FRAG_BIT_TEX0);
    }
-   else if (!(vertexProgram || vertexShader) ||
-            !ctx->VertexProgram._Current) {
+   else if (!(vertexProgram || vertexShader)) {
       /* Fixed function vertex logic */
       /* _NEW_ARRAY */
       GLbitfield varying_inputs = ctx->varying_vp_inputs;