Fix 8->16 bit output conversion overflow error

The I2S outputs were converting samples in the buffer passed in
from u8->s16 on every call.  When the I2S was full, however, the
sample would not get pushed and the output function would be
called at a later time with the same buffer...which was already
at 16 bits.  It'd be converted again and turned into a high
frequency noise on the output.

Now only modify a copy of the input sample in I2S and I2SNoDAC.
Fixes the conversion problem competely.

Fixes earlephilhower#2

README.md

@@ -111,7 +111,7 @@ AudioFileSourceHTTPStream:  Simple implementation of a streaming HTTP reader for
 ## AudioFileSourceBuffer - Double buffering, useful for HTTP streams
 AudioFileSourceBuffer is an input source that simpy adds an additional RAM buffer of the output of any other AudioFileSource.  This is particularly useful for web streaming where you need to have 1-2 packets in memory to ensure hiccup-free playback.
 
-Simply create your standard input file source, create the buffer with the original source as its input, and pass this buffer object to the generator.
+Create your standard input file source, create the buffer with the original source as its input, and pass this buffer object to the generator.
 ````
 ...
 AudioGeneratorMP3 *mp3;

src/AudioOutputI2S.cpp

@@ -168,25 +168,29 @@ bool AudioOutputI2S::begin()
 
 bool AudioOutputI2S::ConsumeSample(int16_t sample[2])
 {
-  MakeSampleStereo16( sample );
+  int16_t ms[2];
+
+  ms[0] = sample[0];
+  ms[1] = sample[1];
+  MakeSampleStereo16( ms );
 
   if (this->mono) {
     // Average the two samples and overwrite
-    uint32_t ttl = sample[LEFTCHANNEL] + sample[RIGHTCHANNEL];
-    sample[LEFTCHANNEL] = sample[RIGHTCHANNEL] = (ttl>>1) & 0xffff;
+    int32_t ttl = ms[LEFTCHANNEL] + ms[RIGHTCHANNEL];
+    ms[LEFTCHANNEL] = ms[RIGHTCHANNEL] = (ttl>>1) & 0xffff;
   }
 #ifdef ESP32
   uint32_t s32;
   if (output_mode == INTERNAL_DAC) {
-    int16_t l = Amplify(sample[LEFTCHANNEL]) + 0x8000;
-    int16_t r = Amplify(sample[RIGHTCHANNEL]) + 0x8000;
+    int16_t l = Amplify(ms[LEFTCHANNEL]) + 0x8000;
+    int16_t r = Amplify(ms[RIGHTCHANNEL]) + 0x8000;
     s32 = (r<<16) | (l&0xffff);
   } else {
-    s32 = ((Amplify(sample[RIGHTCHANNEL]))<<16) | (Amplify(sample[LEFTCHANNEL]) & 0xffff);
+    s32 = ((Amplify(ms[RIGHTCHANNEL]))<<16) | (Amplify(ms[LEFTCHANNEL]) & 0xffff);
   }
   return i2s_push_sample((i2s_port_t)portNo, (const char *)&s32, 0);
 #else
-  uint32_t s32 = ((Amplify(sample[RIGHTCHANNEL]))<<16) | (Amplify(sample[LEFTCHANNEL]) & 0xffff);
+  uint32_t s32 = ((Amplify(ms[RIGHTCHANNEL]))<<16) | (Amplify(ms[LEFTCHANNEL]) & 0xffff);
   return i2s_write_sample_nb(s32); // If we can't store it, return false.  OTW true
 #endif
 }

src/AudioOutputI2SNoDAC.cpp

@@ -80,11 +80,14 @@ void AudioOutputI2SNoDAC::DeltaSigma(int16_t sample[2], uint32_t dsBuff[8])
 
 bool AudioOutputI2SNoDAC::ConsumeSample(int16_t sample[2])
 {
-  MakeSampleStereo16( sample );
+  int16_t ms[2];
+  ms[0] = sample[0];
+  ms[1] = sample[1];
+  MakeSampleStereo16( ms );
 
   // Make delta-sigma filled buffer
   uint32_t dsBuff[8];
-  DeltaSigma(sample, dsBuff);
+  DeltaSigma(ms, dsBuff);
 
   // Either send complete pulse stream or nothing
 #ifdef ESP32