fix: resample key images with a real filter instead of nearest-neighbour

[constrictor]

Fixes #61.

convert_image_with_format resampled every key image with FilterType::Nearest, which discards the antialiasing of whatever the caller drew. On a Stream Deck + that is 144 → 120, a 6:5 ratio, so it dropped every sixth row and column and point-sampled soft edges instead of averaging them — stair-stepped diagonals, strokes whose width visibly alternates along their length, and small text reduced to mush.

This switches to Lanczos3 and adds an early-out when the image already matches the key size.

The early-out is worth having on its own: resize_exact runs the sampler unconditionally, so a caller that already hands over a correctly-sized image was paying for a full resample to get its own pixels back. That case is now free, which also gives callers a way to opt out of the filter entirely by pre-scaling.

Triangle would also be a large improvement if Lanczos3 seems too heavy — happy to switch.

Correction to an earlier version of this description, prompted by @Terrorwolf01 in #61: I originally justified the filter choice with "key images are at most 200×200". That number was not derived from anything. The real ceiling in Kind::key_image_format() is 120×120, and it is the wrong bound regardless — this function is public and also takes lcd_image_format(), which reaches 800×100 and 100×1200. Cost is really driven by the source size, which is unbounded through the public API. Measured on image 0.25.10, release build:

case	source → dest	Nearest	Triangle	Lanczos3
Plus key	144×144 → 120×120	133 µs	188 µs	320 µs
Mk2 key	144×144 → 72×72	54 µs	88 µs	182 µs
XL key, large source	512×512 → 96×96	148 µs	597 µs	1507 µs
key, very large source	1024×1024 → 120×120	286 µs	2078 µs	6852 µs
Plus LCD strip	900×112 → 800×100	746 µs	1006 µs	1674 µs
PlusXl LCD	112×1400 → 100×1200	1088 µs	1512 µs	2573 µs

For a host rendering at roughly key size, Lanczos3 costs ~190 µs more per update than Nearest. The outlier is a large source image: 1024×1024 → 120×120 is 24× Nearest. On that evidence Triangle is arguably the better default — ~3.3× cheaper there, and visually near-identical to Lanczos3 at the ratios that motivated this. Say the word and I'll switch it.

Testing

I found this writing an OpenDeck plugin that draws its key faces as SVG, against a Stream Deck +. Rendering the same source through the full pipeline twice — including both JPEG passes, changing only the filter — the Nearest output matches what the panel shows and the Lanczos3 output is clean, with no change to the artwork.

I could not run cargo check on the crate itself here: hidapi needs libudev headers that aren't installed on this machine. I type-checked the changed expression against image 0.25.10 with the same imports images.rs has, and rustfmt --check on src/images.rs is clean. Worth a second pair of eyes on the build.

[nekename]

I've commented on the issue. In the future, please only open a PR and not an issue, since the discussion can be tracked in a single place without increasing maintenance overhead.