Remove another memory copy with compression.

I considered using composite byte buffers but they would have added GC
overhead and would've been incompatible with any native code we added
unless special care was taken.

proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftCompressEncoder.java

@@ -17,22 +17,14 @@ public class MinecraftCompressEncoder extends MessageToByteEncoder<ByteBuf> {
 
     @Override
     protected void encode(ChannelHandlerContext ctx, ByteBuf msg, ByteBuf out) throws Exception {
-        if (msg.readableBytes() <= threshold) {
+        int uncompressed = msg.readableBytes();
+        if (uncompressed <= threshold) {
             // Under the threshold, there is nothing to do.
             ProtocolUtils.writeVarInt(out, 0);
             out.writeBytes(msg);
-            return;
-        }
-
-        // in other words, see if a plain 8KiB buffer fits us well
-        ByteBuf compressedBuffer = ctx.alloc().buffer(8192);
-        try {
-            int uncompressed = msg.readableBytes();
-            compressor.deflate(msg, compressedBuffer);
+        } else {
             ProtocolUtils.writeVarInt(out, uncompressed);
-            out.writeBytes(compressedBuffer);
-        } finally {
-            compressedBuffer.release();
+            compressor.deflate(msg, out);
         }
     }
 

proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftVarintFrameDecoder.java

@@ -10,7 +10,7 @@ import java.util.List;
 public class MinecraftVarintFrameDecoder extends ByteToMessageDecoder {
     @Override
     protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
-        if (in.readableBytes() < 1) {
+        if (!in.isReadable()) {
             return;
         }