Fix y-axis offset in feSpotLight transform_light_source#1052
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`transform_light_source` was subtracting `region.x()` from the y-component of both the light source position and its `points_at` target in the `SpotLight` branch. The result is that whenever the filter region origin has `x != y`, the spotlight is offset by `region.x() - region.y()` pixels on the y axis, producing mis-rendered lighting. The adjacent `PointLight` branch uses `region.y()` correctly, so this is a copy-paste bug introduced when the SpotLight branch was derived from PointLight.
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transform_light_sourcewas subtractingregion.x()from the y-component of bothlight.yandlight.points_at_yin theSpotLightbranch (crates/resvg/src/filter/mod.rs:1079, 1085). Whenever the filter region origin hasx != y, the spotlight is offset on the y axis byregion.x() - region.y()pixels, producing mis-rendered specular/diffuse lighting.The adjacent
PointLightbranch already usesregion.y()for the y subtraction (line 1070), so the SpotLight branch is a copy-paste bug. This change brings SpotLight in line with PointLight and the SVG filter spec.Existing filter tests (all 249) still pass; they happen to use filter regions anchored at the origin so the bug was not surfaced in the snapshot suite.