Vendor raptor source files (was embedded gitlink)

Tree vendored at upstream 0019c5dd; origin + license recorded in
raptor/VENDOR.txt. Windows SDL dev packages excluded via .gitignore.

raptor/include/SDL2-devel-2.28.2-VC/SDL2-2.28.2/README-SDL.txt

@@ -0,0 +1,13 @@
+
+Please distribute this file with the SDL runtime environment:
+
+The Simple DirectMedia Layer (SDL for short) is a cross-platform library
+designed to make it easy to write multi-media software, such as games
+and emulators.
+
+The Simple DirectMedia Layer library source code is available from:
+https://www.libsdl.org/
+
+This library is distributed under the terms of the zlib license:
+http://www.zlib.net/zlib_license.html
+