Add sdl3_loadso.odin

vendor/sdl3/sdl3_loadso.odin

@@ -0,0 +1,10 @@
+package sdl3
+
+SharedObject :: struct {}
+
+@(default_calling_convention="c", link_prefix="SDL_", require_results)
+foreign lib {
+	LoadObject   :: proc(sofile: cstring) -> ^SharedObject ---
+	LoadFunction :: proc(handle: ^SharedObject, name: cstring) -> FunctionPointer ---
+	UnloadObject :: proc(handle: ^SharedObject) ---
+}