implement TSkybox and use setColor method to set the background color, rather than the image

thermion_dart/native/include/c_api/TSkybox.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include "APIExport.h"
+#include "APIBoundaryTypes.h"
+
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+EMSCRIPTEN_KEEPALIVE void Skybox_setColor(TSkybox* tSkybox, double r, double g, double b, double a);
+
+#ifdef __cplusplus
+}
+#endif
+