aacenc: Fix the conditions under which 3GPP pre-echo control is run.

According to the 3GPP spec:
"Thus the pre-echo control is inactive for the first short window (but
not all short windows in a short frame) after a start block and for
all frames with a stop window sequence."

Currently, pre-echo control is only run when the current frame is not
a short frame, and the previous frame is not a short frame.

patch by Nathan Caldwell saintdev (at) gmail

Originally committed as revision 25587 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/aacpsy.c

@@ -420,7 +420,7 @@ static void psy_3gpp_analyze(FFPsyContext *ctx, int channel,
             band[g].thr = FFMAX(band[g].thr, band[g+1].thr * coeffs->spread_hi [g]);
         for (g = 0; g < num_bands; g++) {
             band[g].thr_quiet = FFMAX(band[g].thr, coeffs->ath[g]);
-            if (wi->num_windows != 8 && wi->window_type[1] != EIGHT_SHORT_SEQUENCE)
+            if (!(wi->window_type[0] == LONG_STOP_SEQUENCE || (wi->window_type[1] == LONG_START_SEQUENCE && !w)))
                 band[g].thr_quiet = FFMAX(PSY_3GPP_RPEMIN*band[g].thr_quiet,
                                           FFMIN(band[g].thr_quiet,
                                           PSY_3GPP_RPELEV*pch->prev_band[w+g].thr_quiet));