vulkan: 64 MiB VMA blocks; device memory usage without VMA block slack

Title 1's texture cache deleted 1,400-2,600 textures per 600 frames
and created them again: device memory sat at the collector's critical
line, but ~38% of it was empty space in 256 MiB VMA blocks. Freeing an
image returns that space only once its whole block is empty, so the
caches kept collecting without the usage dropping (one mass purge
deleted 4,300 textures).

- 64 MiB blocks on discrete GPUs too (as on integrated ones).
- GetDeviceMemoryUsage subtracts the unused bytes in VMA blocks, so
  each eviction counts at once.

Now 0-224 deletions per 600 frames, ~4,400 textures resident, and
driver usage 6.24 GB of a 6.97 GB budget.

0 files changed, 0 insertions, 0 deletions