texture_cache: keep the allocator's block slack out of the usable VRAM

The collector's usage leaves out the unused part of VMA's blocks, but
the driver counts it and a heap at its budget cannot give it to a new
image. On an RX 7800 XT that slack was 1.06 GiB, the whole 1 GiB margin:
the heap sat at its budget for a minute while the collector saw room,
then images stopped fitting and the process died. Reserve the slack
(capped at 1.5 GiB) on top of the margin: at once when it grows, slowly
when it shrinks, so eviction leaving partly used blocks doesn't chase
the threshold down.

Assisted-by: Claude Sonnet 5.5 (diagnosis and code)
(cherry picked from commit 73fd5a489ea28226fa16a55f5008e622d631b54c)

0 files changed, 0 insertions, 0 deletions