Implement HDRI skybox support

assets/shaders/skybox.frag

@@ -17,6 +17,7 @@ vec2 sample_spherical_map(vec3 v) {
 
 void main() {
     vec2 uv = sample_spherical_map(normalize(direction));
+    uv.y = 1.0 - uv.y;
     vec3 color = texture(equirect, uv).rgb;
     frag_color = vec4(color, 1.0);
 }