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.txt

@@ -0,0 +1,21 @@
+
+                         Simple DirectMedia Layer
+
+                                  (SDL)
+
+                                Version 2.0
+
+---
+https://www.libsdl.org/
+
+Simple DirectMedia Layer is a cross-platform development library designed
+to provide low level access to audio, keyboard, mouse, joystick, and graphics
+hardware via OpenGL and Direct3D. It is used by video playback software,
+emulators, and popular games including Valve's award winning catalog
+and many Humble Bundle games.
+
+More extensive documentation is available in the docs directory, starting
+with README.md
+
+Enjoy!
+	Sam Lantinga				(slouken@libsdl.org)