Add rect_tx_ext experimental flag

An experiment on 1:4/4:1 transform, which will be a sub-experiment
of ext_tx+rect_tx experiment

Change-Id: I37c41227f83c51ab11194007226a2d6bcd6ff74f

configure

@@ -249,6 +249,7 @@ EXPERIMENT_LIST="
     cdef
     var_tx
     rect_tx
+    rect_tx_ext
     tpl_mv
     dual_filter
     convolve_round