Fix completely broken SHs lol!

My reordering code was effectively only picking up the SH1 coefficient instead of all 15 ones.
So while the overall "color" (SH0) was correct, the directional aspect was, uhh, "not great".
And with that, fix the SH direction axis swapping to be correct.

Author: aras-p

Assets/Scripts/GaussianSplatRenderer.cs

@@ -99,9 +99,9 @@ static unsafe void ReorderSHs(int splatCount, float* data)
         {
             for (int j = 0; j < shCount; ++j)
             {
-                tmp[j * 3 + 0] = data[idx];
-                tmp[j * 3 + 1] = data[idx + shCount];
-                tmp[j * 3 + 2] = data[idx + shCount * 2];
+                tmp[j * 3 + 0] = data[idx + j];
+                tmp[j * 3 + 1] = data[idx + j + shCount];
+                tmp[j * 3 + 2] = data[idx + j + shCount * 2];
             }
 
             for (int j = 0; j < shCount * 3; ++j)

Assets/Shaders/RenderGaussianSplats.shader

@@ -65,13 +65,14 @@ static const float SH_C3[] = {
 half3 ShadeSH(InputSplat splat, float3 dir)
 {
 	dir *= -1;
+	dir.z *= -1;
 
     float x = dir.x, y = dir.y, z = dir.z;
 
     // ambient band
     half3 res = SH_C0 * splat.sh0;
     // 1st degree
-    res += - splat.sh1 * (y * SH_C1) + splat.sh2 * (z * SH_C1) - splat.sh3 * (x * SH_C1);
+    res += SH_C1 * (-splat.sh1 * y + splat.sh2 * z - splat.sh3 * x);
     // 2nd degree
     float xx = x * x, yy = y * y, zz = z * z;
     float xy = x * y, yz = y * z, xz = x * z;
@@ -89,7 +90,7 @@ half3 ShadeSH(InputSplat splat, float3 dir)
 		(SH_C3[3] * z * (2 * zz - 3 * xx - 3 * yy)) * splat.sh12 +
 		(SH_C3[4] * x * (4 * zz - xx - yy)) * splat.sh13 +
 		(SH_C3[5] * z * (xx - yy)) * splat.sh14 +
-		(SH_C3[6] * x * (xx - 3.0f * yy)) * splat.sh15;
+		(SH_C3[6] * x * (xx - 3 * yy)) * splat.sh15;
     return max(res + 0.5, 0);
 }