AppConfig: Disable libGL forwarding for steamwebhelper

This app bypasses the glX functions exported by libGL and instead sends GLX
requests directly via xcb. FEX won't support forwarding this usage pattern in
the foreseeable future.

Data/AppConfig/steamwebhelper.json

@@ -0,0 +1,6 @@
+{
+  "Comment": "Bypasses libGL's glX and instead sends GLX requests directly via xcb",
+  "ThunksDB": {
+    "GL": 0
+  }
+}