dnd_glfw: headers, style and macOS fix

Add SPDX headers. Reformat and tidy dnd_glfw.h (includes, spacing, inline function layout, and minor Windows DropTarget refactor) without changing public API. On macOS, stop registering NSFilenamesPboardType and adjust cocoaView bridging to use manual cast/release (fixes retain/release semantics). Small adjustment to GLFW drop-callback setup/assignment formatting.

Signed-off-by: Vlad (Kuzmin) Erium <libalias@gmail.com>
Signed-off-by: Vlad <shaamaan@gmail.com>

Author: ssh4net

src/imiv/external/dnd_glfw/LICENSE

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2025-2025 Erium Vladlen
+Copyright (c) 2025-2026 Erium Vladlen
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal