If Y/AC is 0 (but lossless=0), only allow coding DCT_DCT

This is consistent with what libaom does. Should fix #175.

src/env.h

@@ -127,9 +127,13 @@ static inline enum TxfmTypeSet get_ext_txtp_set(const enum RectTxfmSize tx,
                                                 const Av1FrameHeader *const hdr,
                                                 const int seg_id)
 {
+    if (!hdr->segmentation.qidx[seg_id]) {
         if (hdr->segmentation.lossless[seg_id]) {
             assert(tx == (int) TX_4X4);
             return TXTP_SET_LOSSLESS;
+        } else {
+            return TXTP_SET_DCT;
+        }
     }
 
     const TxfmInfo *const t_dim = &dav1d_txfm_dimensions[tx];