Title 1's terrain virtual textures are 8192x8192x68 ASTC 8x8 sRGB arrays with
12 levels: 1.5 GB in guest memory (fully backed) but 23 GB each as the RGBA8
Eden decodes ASTC to, so they were created unbacked and the near terrain
sampled black. A GPU-decoded ASTC 2D array over 1 GiB decoded now keeps only
the levels from the first one that fits in 512 MiB (here 3: 1024x1024, 380
MiB); host mip 0 is that guest level. Uploads read each layer's tail of the
mip chain into a compact buffer and decode only those levels, views shift
their level range, and these images are never downloaded. Normalized
coordinates and derivative LOD address the smaller image correctly; the
nearest terrain is lower resolution than on the console.
Also logs how unbacked images are used (views, render targets, blits, DMA).
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