Add skybox rendering support

resources/shaders/skybox.frag

@@ -0,0 +1,22 @@
+#version 330 core
+
+in vec3 direction;
+
+out vec4 frag_color;
+
+uniform sampler2D equirect;
+
+const vec2 inv_atan = vec2(0.15915494309, 0.31830988618);
+
+vec2 sample_spherical_map(vec3 v) {
+    vec2 uv = vec2(atan(v.z, v.x), asin(v.y));
+    uv *= inv_atan;
+    uv += 0.5;
+    return uv;
+}
+
+void main() {
+    vec2 uv = sample_spherical_map(normalize(direction));
+    vec3 color = texture(equirect, uv).rgb;
+    frag_color = vec4(color, 1.0);
+}