avcodec/on2avc: fix regression on icc since 5495c7f2

Should fix the regression, and also speeds up table generation.
Tables tested on GNU/Linux+clang: they are identical to the ones prior
to 5495c7f2. ff_exp10 caused one slight change in one entry, 50000 became
50001 due to somewhat incorrect rounding.

Untested on ICC; passes FATE on GNU/Linux+gcc.
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavcodec/on2avc.c

@@ -912,7 +912,23 @@ static av_cold void on2avc_free_vlcs(On2AVCContext *c)
 static av_cold int on2avc_decode_init(AVCodecContext *avctx)
 {
     On2AVCContext *c = avctx->priv_data;
-    int i;
+    int i, ph;
+    /* 10^(i*0.1) for 0 <= i < 10 */
+    /* TODO: possibly statically allocate scale_tab; this may help with FATE
+     * and reproducibility if the binary size is not impacted much */
+    static const double exp10_lut[] = {
+        1,
+        1.2589254117941673,
+        1.5848931924611136,
+        1.9952623149688795,
+        2.5118864315095806,
+        3.1622776601683795,
+        3.9810717055349727,
+        5.0118723362727229,
+        6.3095734448019334,
+        7.9432823472428158,
+    };
+    int64_t exp10_base = 10;
 
     if (avctx->channels > 2U) {
         avpriv_request_sample(avctx, "Decoding more than 2 channels");
@@ -934,10 +950,23 @@ static av_cold int on2avc_decode_init(AVCodecContext *avctx)
         av_log(avctx, AV_LOG_WARNING,
                "Stereo mode support is not good, patch is welcome\n");
 
-    for (i = 0; i < 20; i++)
+
+    /* Fast and more accurate way of doing for (i = 0; i < 20; i++)
         c->scale_tab[i] = ceil(ff_exp10(i * 0.1) * 16) / 32;
     for (; i < 128; i++)
-        c->scale_tab[i] = ceil(ff_exp10(i * 0.1) * 0.5);
+        c->scale_tab[i] = ceil(ff_exp10(i * 0.1) * 0.5); */
+    for (i = 0; i < 10; i++) {
+        c->scale_tab[i] = ceil(exp10_lut[i] * 16) / 32;
+        c->scale_tab[i+10] = ceil(exp10_lut[i] * 160) / 32;
+    }
+
+    for (i = 20, ph = 0; i < 128; i++, ph++) {
+        if (i % 10 == 0) {
+            exp10_base *= 10;
+            ph = 0;
+        }
+        c->scale_tab[i] = ceil(exp10_base * exp10_lut[ph] * 0.5);
+    }
 
     if (avctx->sample_rate < 32000 || avctx->channels == 1)
         memcpy(c->long_win, ff_on2avc_window_long_24000,