Fix readTextureToTexelViews for cubemaps (#4177)

The issue was that the code that reads back the texture
uses textureGather for cubemaps. It has to use textureGather
because it's the only way to load from cubemaps with integer
formats. It was computing a texel coord and then doing the
common conversion to normalized texture coord

normalizedCoord = (integerTexelCoord + 0.5) / dimension

But, for textureGather, 0.5 is the center of the texel and so
it's an edge case. The gather could go in any direction from
the center. Moving it off center should fix that issue.

src/webgpu/shader/execution/expression/call/builtin/texture_utils.ts

@@ -2961,7 +2961,11 @@ export async function readTextureToTexelViews(
       fn textureLoadCubeAs2DArray(tex: texture_cube<${componentType}>, coord: vec2u, layer: u32) -> ${resultType} {
         // convert texel coord normalized coord
         let size = textureDimensions(tex, 0);
-        let uv = (vec2f(coord) + 0.5) / vec2f(size.xy);
+
+        // Offset by 0.75 instead of the more common 0.5 for converting from texel to normalized texture coordinate
+        // because we're using textureGather. 0.5 would indicate the center of a texel but based on precision issues
+        // the "gather" could go in any direction from that center. Off center it should go in an expected direction.
+        let uv = (vec2f(coord) + 0.75) / vec2f(size.xy);
 
         // convert uv + layer into cube coord
         let cubeCoord = faceMat[layer] * vec3f(uv, 1.0);