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/docs/README-platforms.md

@@ -0,0 +1,8 @@
+Platforms
+=========
+
+We maintain the list of supported platforms on our wiki now, and how to
+build and install SDL for those platforms:
+
+    https://wiki.libsdl.org/Installation
+