Add anisotropic Gaussian blur

[theashraf]

FilterPrimitive::GaussianBlur now takes a standard deviation per axis (std_deviation_x, std_deviation_y), matching SVG's two-value stdDeviation. Equal values behave exactly as before; a zero deviation leaves that axis untouched. This resolves the two TODOs in filter_effects.rs and transform_blur_params.

Approach

• Per-axis plan. GaussianBlur carries a decimation count and kernel per axis. The finest decimation levels are shared between the axes; once the smaller axis runs out of levels only the larger one keeps being halved (decimation_levels). DecimationSizer tracks the axes separately through downscale_axes / upscale_axes.
• CPU. Single-axis downscale/upscale, and the convolution pass of an identity kernel is skipped.
• GPU. New DOWNSCALE_X/Y and UPSCALE_X/Y passes; downscale/upscale in scale.wesl take an axis mask. The filter data grows from 48 to 64 bytes to carry a second kernel, and the header packs the y-axis decimation and tap counts in the previously reserved bits. Drop shadows still blur both axes with one kernel, so their layout is unchanged.
• Transforms. An anisotropic blur is scaled through the covariance of the Gaussian: a 90° rotation swaps the axes and a non-uniform scale scales each axis on its own. The off-diagonal term of a rotation that is not a multiple of 90° cannot be represented by an axis-aligned kernel and is dropped. Uniform blurs keep the previous averaged scale factor.

Tests

• Unit tests for the per-axis plan, the sizer with mixed axes, and the transform math.
• CPU tests that a single-axis blur leaves the other axis bit-exact, and that an x-only blur is the transpose of a y-only blur.
• GPU pass-plan tests for single-axis and mixed-axis blurs.
• New snapshots: filter_gaussian_blur_horizontal, _vertical, _anisotropic, _anisotropic_rotated, _anisotropic_scaled. Existing snapshots are unchanged.

Breaking changes

FilterPrimitive::GaussianBlur { std_deviation } is now { std_deviation_x, std_deviation_y }, and GaussianBlur::new takes both deviations.