Reintroduce projective blinding

src/ecmult_gen.h

@@ -109,6 +109,10 @@ typedef struct {
      * ecmult_gen_impl.h for more details. */
     secp256k1_scalar scalar_offset;
     secp256k1_ge ge_offset;
+
+    /* Factor used for projective blinding. This value is used to rescale the Z
+     * coordinate of the first table lookup. */
+    secp256k1_fe proj_blind;
 } secp256k1_ecmult_gen_context;
 
 static void secp256k1_ecmult_gen_context_build(secp256k1_ecmult_gen_context* ctx);