added skybox asset

assets/shaders/skybox.frag

@@ -0,0 +1,12 @@
+#version 450
+
+layout (set = 0, binding = 0) uniform samplerCube skybox;
+
+layout(location = 0) in vec3 texCoords;
+
+layout(location = 0) out vec4 outColor;
+
+void main()
+{
+  outColor = texture(skybox, texCoords);
+}