Fix orientation of cubemap sample, add clearer attribution

Fixes 341

public/assets/img/cubemap/credit.txt

@@ -0,0 +1 @@
+Cubemap image available under a Creative Commons Attribution 3.0 Unported License at https://www.humus.name/index.php?page=Textures&ID=58

sample/cubemap/meta.ts

@@ -1,7 +1,7 @@
 export default {
   name: 'Cubemap',
   description:
-    'This example shows how to render and sample from a cubemap texture.',
+    'This example shows how to render and sample from a cubemap texture. Cubemap image available under a Creative Commons Attribution 3.0 Unported License at https://www.humus.name/index.php?page=Textures&ID=58',
   filename: __DIRNAME__,
   sources: [
     { path: 'main.ts' },

sample/cubemap/sampleCubemap.frag.wgsl

@@ -7,8 +7,12 @@ fn main(
   @location(1) fragPosition: vec4f
 ) -> @location(0) vec4f {
   // Our camera and the skybox cube are both centered at (0, 0, 0)
-  // so we can use the cube geomtry position to get viewing vector to sample the cube texture.
-  // The magnitude of the vector doesn't matter.
+  // so we can use the cube geometry position to get viewing vector to sample
+  // the cube texture. The magnitude of the vector doesn't matter.
   var cubemapVec = fragPosition.xyz - vec3(0.5);
+  // When viewed from the inside, cubemaps are left-handed (z away from viewer),
+  // but common camera matrix convention results in a right-handed world space
+  // (z toward viewer), so we have to flip it.
+  cubemapVec.z *= -1;
   return textureSample(myTexture, mySampler, cubemapVec);
 }