mesa: Track the vertex program active at BeginTransformFeedback() time.

The next few patches will use this for API error checking.

All of the drivers appear to CALLOC_STRUCT transform feedback objects,
so this should be properly NULL initialized on creation.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/main/mtypes.h

@@ -1717,6 +1717,12 @@ struct gl_transform_feedback_object
                                 at least once? */
    GLboolean EverBound; /**< Has this object been bound? */
 
+   /**
+    * The shader program active when BeginTransformFeedback() was called.
+    * When active and unpaused, this equals ctx->Shader.CurrentVertexProgram.
+    */
+   struct gl_shader_program *shader_program;
+
    /**
     * GLES: if Active is true, remaining number of primitives which can be
     * rendered without overflow.  This is necessary to track because GLES

src/mesa/main/transformfeedback.c

@@ -406,6 +406,8 @@ _mesa_BeginTransformFeedback(GLenum mode)
       obj->GlesRemainingPrims = max_vertices / vertices_per_prim;
    }
 
+   obj->shader_program = ctx->Shader.CurrentVertexProgram;
+
    assert(ctx->Driver.BeginTransformFeedback);
    ctx->Driver.BeginTransformFeedback(ctx, mode, obj);
 }