Add simple API version fetch function.

* Plugins/apps that build against the header (and so get the
  RENDERDOC_API_VERSION macro) can call this to verify that they're running
  against compatible API functions. If the versions don't match, the app
  should immediately quit.

renderdoc/replay/entry_points.cpp

@@ -84,6 +84,12 @@ extern "C" RENDERDOC_API void RENDERDOC_CC Maths_CameraFPSLook(const FloatVector
 	right->z = r.z;
 }
 
+extern "C" RENDERDOC_API
+int RENDERDOC_CC RENDERDOC_GetAPIVersion()
+{
+	return RENDERDOC_API_VERSION;
+}
+
 extern "C" RENDERDOC_API
 void RENDERDOC_CC RENDERDOC_LogText(const wchar_t *text)
 {