element/image: include the fit mode in the cache key

[UncleJ4ck]

the cached texture bakes in the fit mode, but the cache key was just the path (plus the svg/icon size), so two image elements sharing a path with different fit modes collided on one cache entry and the second drew with the first's fit. shows up with hyprpaper putting one image on two monitors with different fit_mode.

CAssetCache::get matches by source() equality and getCacheString omitted the fit mode, so the second element reused the first's texture (fitMode is baked in at uploadTexture). appending sc<int>(data.fitMode) to the key gives each fit mode its own entry while keeping in-flight dedup for elements that share path and fit mode.

STextureData carries only resource and fitMode, so that one field is the whole class; rounding and alpha are draw-time shader uniforms and need no key change.

added a unit test asserting the key differs across fit modes and matches for the same one. alternative to #96, which guarded the cache hit instead of the key.

build-tested + unit suite (37/37); the two-monitor pixel repro needs two outputs.