Don't copy full transformed buf during every transform

The resulting modified buf is the exact same as before

common/src/main/java/com/viaversion/viaversion/connection/UserConnectionImpl.java

@@ -351,12 +351,25 @@ public class UserConnectionImpl implements UserConnection {
             throw cancelSupplier.apply(ex);
         }
 
-        ByteBuf transformed = wrapper.allocateOutputBuffer();
+        writeToBuffer(wrapper, buf);
+    }
+
+    private void writeToBuffer(final PacketWrapperImpl wrapper, final ByteBuf buf) {
+        // Instead of allocating a possible unnecessarily large buffer to write the wrapper contents to,
+        // only allocate the remaining bytes and write the rest to the original buf's head directly.
+        final int remainingBytes = buf.readableBytes();
+        final ByteBuf remainingBuf = buf.alloc().buffer(remainingBytes);
         try {
-            wrapper.writeToBuffer(transformed);
-            buf.clear().writeBytes(transformed);
+            // Copy before modifying the buffer
+            remainingBuf.writeBytes(buf, remainingBytes);
+
+            // Reset indexes, write wrapper contents, then the unread bytes
+            buf.readerIndex(0);
+            buf.writerIndex(0);
+            wrapper.writeProcessedValues(buf);
+            buf.writeBytes(remainingBuf);
         } finally {
-            transformed.release();
+            remainingBuf.release();
         }
     }
 

common/src/main/java/com/viaversion/viaversion/protocol/packet/PacketWrapperImpl.java

@@ -222,6 +222,13 @@ public class PacketWrapperImpl implements PacketWrapper {
 
     @Override
     public void writeToBuffer(ByteBuf buffer) throws InformativeException {
+        writeProcessedValues(buffer);
+        if (inputBuffer != null) {
+            buffer.writeBytes(inputBuffer);
+        }
+    }
+
+    public void writeProcessedValues(ByteBuf buffer) throws InformativeException {
         if (id != -1) {
             Types.VAR_INT.writePrimitive(buffer, id);
         }
@@ -238,7 +245,6 @@ public class PacketWrapperImpl implements PacketWrapper {
                 throw createInformativeException(e, packetValue.type(), i);
             }
         }
-        writeRemaining(buffer);
     }
 
     private InformativeException createInformativeException(final Exception cause, final Type<?> type, final int index) {
@@ -264,12 +270,6 @@ public class PacketWrapperImpl implements PacketWrapper {
         packetValues.clear();
     }
 
-    private void writeRemaining(ByteBuf output) {
-        if (inputBuffer != null) {
-            output.writeBytes(inputBuffer);
-        }
-    }
-
     @Override
     public void send(Class<? extends Protocol> protocol, boolean skipCurrentPipeline) throws InformativeException {
         send0(protocol, skipCurrentPipeline, true);
@@ -386,7 +386,7 @@ public class PacketWrapperImpl implements PacketWrapper {
         }
     }
 
-    public ByteBuf allocateOutputBuffer() {
+    private ByteBuf allocateOutputBuffer() {
         if (inputBuffer == null) {
             return user().getChannel().alloc().buffer();
         }