texture_cache: keep Switch 2 VRAM use under the driver's budget

Mario Kart World filled the GPU after a few minutes on 8 GB cards, and on
16 GB cards with uncompressed ASTC: textures were created without memory
(garbage), then the emulator crashed. The Switch 2 collector thresholds
came from the sum of all heap budgets, but on AMD the host-visible
device-local heap is the same VRAM, so the critical threshold ended above
the real budget and the collector never kicked in. Unbacked images were
also counted at their full size.

Thresholds now follow the main device-local heap's budget, re-read every
30 frames, and images count the memory they really have. Switch 2 titles
on GPUs under 12 GiB re-compress ASTC to BC3 for the session when it is
set to Uncompressed (EDEN_NX2_KEEP_ASTC=1 opts out), and
EDEN_NX2_VRAM_LIMIT_MIB simulates a smaller GPU for testing.

Assisted-by: Claude Opus 5.5 (diagnosis and code)

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