cache_size_bytes() does a full stat() of every file in the cache
directory. It was called on every image load and every infinite
scroll drain. Now skipped if less than 30 seconds since the last
check.
Also replace QPixmap with QImageReader in image_dimensions() to
read width/height from the file header without decoding the full
image into memory.
behavior change: cache eviction checks run at most once per 30s
instead of on every image load. Library image dimensions are read
via QImageReader (header-only) instead of QPixmap (full decode).
pax
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