feature: Add lc3_encoder_disable_ltpf

LTPF requires a lot of processing power, so disabling it might be
required for embedded low-performance devices.

src/lc3.c

@@ -301,7 +301,7 @@ static void analyze(struct lc3_encoder *encoder,
 
     bool att = lc3_attdet_run(dt, sr_pcm, nbytes, &encoder->attdet, xt);
 
-    side->pitch_present =
+    side->pitch_present = !encoder->ltpf_bypass &&
         lc3_ltpf_analyse(dt, sr_pcm, &encoder->ltpf, xt, &side->ltpf);
 
     memmove(xt - nt, xt + (ns-nt), nt * sizeof(*xt));
@@ -313,7 +313,7 @@ static void analyze(struct lc3_encoder *encoder,
     lc3_mdct_forward(dt, sr_pcm, sr, xs, xd, xf);
 
     bool nn_flag = lc3_energy_compute(dt, sr, xf, e);
-    if (nn_flag)
+    if (nn_flag || encoder->ltpf_bypass)
         lc3_ltpf_disable(&side->ltpf);
 
     side->bw = lc3_bwdet_run(dt, sr, e);
@@ -424,6 +424,12 @@ LC3_EXPORT struct lc3_encoder *lc3_setup_encoder(
     return lc3_hr_setup_encoder(false, dt_us, sr_hz, sr_pcm_hz, mem);
 }
 
+LC3_EXPORT void lc3_encoder_disable_ltpf(
+    struct lc3_encoder *encoder)
+{
+    encoder->ltpf_bypass = true;
+}
+
 /**
  * Encode a frame
  */