PCK: Set VERSION_PATCH in header, factor out header magic

Unify pack file version and magic to avoid hardcoded literals.

`version.py` now always includes `patch` even for the first release in
a new stable branch (e.g. 3.2). The public name stays without the patch
number, but `Engine.get_version_info()` already included `patch == 0`,
and we can remove some extra handling of undefined `VERSION_PATCH` this
way.

Co-authored-by: Rémi Verschelde <rverschelde@gmail.com>

core/io/file_access_pack.h

@@ -37,6 +37,11 @@
 #include "core/os/file_access.h"
 #include "core/print_string.h"
 
+// Godot's packed file magic header ("GDPC" in ASCII).
+#define PACK_HEADER_MAGIC 0x43504447
+// The current packed file format version number.
+#define PACK_FORMAT_VERSION 1
+
 class PackSource;
 
 class PackedData {