Use 32-bit tran_low_t values for txmg experiment

The generic transforms (av1_fwd_txfm1d.c) require 32-bit coeffs.
Also, use 32-bit EXTRABIT values.
Fixes a memory corruption when --disable-lowbitdepth.

BUG=aomedia:860

Change-Id: Id8a181702e5bed793b20a16a518eb8a1064f2a21

configure

@@ -572,6 +572,7 @@ post_process_cmdline() {
     enabled new_multisymbol && enable_feature restrict_compressed_hdr
     enabled mfmv && enable_feature frame_marker
     enabled jnt_comp && enable_feature frame_marker
+    enabled txmg && enable_feature highbitdepth
 
     if enabled rawbits && enabled ans; then
       log_echo "rawbits requires not ans, so disabling rawbits"