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

thermion_dart/native/src/c_api/TSkybox.cpp

@@ -0,0 +1,30 @@
+#ifdef __EMSCRIPTEN__
+#include <emscripten.h>
+#endif 
+
+#include "c_api/TSkybox.h"
+
+#include <filament/math/mat4.h>
+#include <filament/Skybox.h>
+
+#include "Log.hpp"
+
+#ifdef __cplusplus
+namespace thermion
+{
+    extern "C"
+    {
+        using namespace filament;
+#endif
+
+        EMSCRIPTEN_KEEPALIVE void Skybox_setColor(TSkybox *tSkybox, double r, double g, double b, double a)
+        {
+            auto *skybox = reinterpret_cast<filament::Skybox *>(tSkybox);
+            skybox->setColor(filament::math::float4 { static_cast<float>(r), static_cast<float>(g), static_cast<float>(b), static_cast<float>(a) } );
+        }
+        
+
+#ifdef __cplusplus
+    }
+}
+#endif