Add system property to permit oversized packets.

This should fix #91, but this is not enabled by default for security reasons. This also does not raise all relevant packet lengths capped by the protocol, although this can be added at a future point in time.

With this system property enabled, Krypton permits packets with 5-byte VarInt prefixes and allows uncompressed packets to be up to 2GiB in size.

src/main/java/me/steinborn/krypton/mod/shared/network/VarintByteDecoder.java

@@ -3,6 +3,10 @@
 import io.netty.util.ByteProcessor;
 
 public class VarintByteDecoder implements ByteProcessor {
+
+    private static final int MAXIMUM_VARINT_BYTE_SIZE = Boolean.getBoolean("krypton.permit-oversized-packets")
+            ? 5 : 3;
+
     private int readVarint;
     private int bytesRead;
     private DecodeResult result = DecodeResult.TOO_SHORT;
@@ -18,7 +22,7 @@ public boolean process(byte k) {
             return false;
         }
         readVarint |= (k & 0x7F) << bytesRead++ * 7;
-        if (bytesRead > 3) {
+        if (bytesRead > MAXIMUM_VARINT_BYTE_SIZE) {
             result = DecodeResult.TOO_BIG;
             return false;
         }

src/main/java/me/steinborn/krypton/mod/shared/network/compression/MinecraftCompressDecoder.java

@@ -12,7 +12,8 @@
 
 public class MinecraftCompressDecoder extends ByteToMessageDecoder {
 
-  private static final int UNCOMPRESSED_CAP = 8 * 1024 * 1024; // 8MiB
+  private static final int UNCOMPRESSED_CAP = Boolean.getBoolean("krypton.permit-oversized-packets")
+          ? Integer.MAX_VALUE : 8 * 1024 * 1024;
 
   private int threshold;
   private final boolean validate;