Fix RenderSettings (de)serialization with Java 16 (fixes #520)

Turns out we were relying on the implementation detail of the File class, and
Java 16 ensures we cannot do that.

src/main/java/com/replaymod/core/utils/FileTypeAdapter.java

@@ -0,0 +1,30 @@
+package com.replaymod.core.utils;
+
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonToken;
+import com.google.gson.stream.JsonWriter;
+
+import java.io.File;
+import java.io.IOException;
+
+public class FileTypeAdapter extends TypeAdapter<File> {
+    @Override
+    public void write(JsonWriter out, File value) throws IOException {
+        out.value(value.getPath());
+    }
+
+    @Override
+    public File read(JsonReader in) throws IOException {
+        String path;
+        if (in.peek() == JsonToken.BEGIN_OBJECT) {
+            in.beginObject();
+            in.nextName();
+            path = in.nextString();
+            in.endObject();
+        } else {
+            path = in.nextString();
+        }
+        return new File(path);
+    }
+}