echo-cancel: Use webrtc's deinterleaved API

This is required to have unequal channel counts on capture in and out
streams, which is needed for beamforming to work. The deinterleaved API
only works with floating point samples.

src/modules/echo-cancel/echo-cancel.h

@@ -66,6 +66,7 @@ struct pa_echo_canceller_params {
             void *apm;
             unsigned int blocksize; /* in frames */
             pa_sample_spec rec_ss, play_ss, out_ss;
+            float *rec_buffer[PA_CHANNELS_MAX], *play_buffer[PA_CHANNELS_MAX]; /* for deinterleaved buffers */
             void *trace_callback;
             bool agc;
             bool first;