Fix up the replay proxy serialising functions

* Also add some compile time asserts to try and help keep them working.

renderdoc/serialise/serialiser.cpp

@@ -1038,6 +1038,16 @@ string ToStrHelper<false, void *>::Get(void* const &el)
 	return tostrBuf;
 }
 
+template<>
+string ToStrHelper<false, int64_t>::Get(const int64_t &el)
+{
+	char tostrBuf[256] = {0};
+
+	StringFormat::snprintf(tostrBuf, 255, "%lld", el);
+
+	return tostrBuf;
+}
+
 template<>
 string ToStrHelper<false, uint64_t>::Get(const uint64_t &el)
 {