Add support for streaming transfers

This change adds onChunkCallback to the request. If defined it will be
called repeatedly with the incoming data. This allows to process data on
the go or write it to disk instead of accumulating the data in memory.

Submitted upstream:
https://github.com/machinezone/IXWebSocket/pull/353

extern/IXWebSocket-11.3.2/ixwebsocket/IXSocket.cpp

@@ -363,12 +363,14 @@ namespace ix
     std::pair<bool, std::string> Socket::readBytes(
         size_t length,
         const OnProgressCallback& onProgressCallback,
+        const OnChunkCallback& onChunkCallback,
         const CancellationRequest& isCancellationRequested)
     {
         std::array<uint8_t, 1 << 14> readBuffer;
-
         std::vector<uint8_t> output;
-        while (output.size() != length)
+        size_t bytesRead = 0;
+
+        while (bytesRead != length)
         {
             if (isCancellationRequested && isCancellationRequested())
             {
@@ -376,12 +378,21 @@ namespace ix
                 return std::make_pair(false, errorMsg);
             }
 
-            size_t size = std::min(readBuffer.size(), length - output.size());
+            size_t size = std::min(readBuffer.size(), length - bytesRead);
             ssize_t ret = recv((char*) &readBuffer[0], size);
 
             if (ret > 0)
             {
-                output.insert(output.end(), readBuffer.begin(), readBuffer.begin() + ret);
+                if (onChunkCallback)
+                {
+                    std::string chunk(readBuffer.begin(), readBuffer.begin() + ret);
+                    onChunkCallback(chunk);
+                }
+                else
+                {
+                    output.insert(output.end(), readBuffer.begin(), readBuffer.begin() + ret);
+                }
+                bytesRead += ret;
             }
             else if (ret <= 0 && !Socket::isWaitNeeded())
             {
@@ -389,7 +400,7 @@ namespace ix
                 return std::make_pair(false, errorMsg);
             }
 
-            if (onProgressCallback) onProgressCallback((int) output.size(), (int) length);
+            if (onProgressCallback) onProgressCallback((int) bytesRead, (int) length);
 
             // Wait with a 1ms timeout until the socket is ready to read.
             // This way we are not busy looping