texture_cache: give images over unwritten sparse blocks no device memory

Mario Kart World creates nine terrain layer pools in sparse regions (four
R8 16384x16384, four RG16 8192x8192, one R16 8192x8192: 2.9 GB of device
memory). In 180 s of title and attract demo not one of their blocks is
written and nothing renders into them, so they hold zeros; on the console
only the few tiles the game maps have memory.

nx2:gpu now publishes which 4 MiB blocks of each sparse region have been
written (nx2.h, a leaf lock the texture cache may take). An image of 32
MiB or more lying in one sparse region with no written block is created
as a sparse resident image with nothing bound: reads return zero, as an
unmapped tile does. The first write into its range, or the first time the
GPU renders into it (that range then counts as written), marks it
remapped, so a backed image replaces it at its next use.

Mario Kart World, 180 s: the collector deletes 0-80 images per 600 frames
instead of thousands, frames over 100 ms per 600 presents drop from 1-9 to
0-1, peak working set 14.7 GB. One render target in a small sparse region
gets replaced once (logged).

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