SVG import: a blur that differs by direction is kept as its SVG filter

A feGaussianBlur or feDropShadow whose blur is not the same in every
direction in the document came in as an even Gaussian Blur or Drop
Shadow of the mean standard deviation. That covers two different
standard deviations (stdDeviation="4 1") and an even one in a skewed or
unevenly scaled user space. An SVG Filters effect in such a user space
is exported live under that transform, so on reopening its blur lost
the difference between directions.

Such a filter is now kept as an SVG Filters effect, which draws it as
it is. A blur in a turned, flipped or evenly scaled user space still
becomes the live effect, and a filter that no SVG Filters effect can
keep still comes in as the nearest even effect, as before.

docs/effects-parity.md

@@ -27,7 +27,7 @@ Part of [target-app-parity.md](target-app-parity.md); open work is in [gaps.md](
 | Pathfinder | Add, Intersect, Exclude, Subtract, Minus Back, Divide, Trim, Merge, Crop, Outline, Hard Mix, Soft Mix, Trap | 13/13 | Trap traps only solid fills (gradients and patterns hide what is below them but don't trap) and spreads each colour's visible area as a whole; the public page gives no default values, so Trap's 0.25 pt, 100% and 40% are VectorCraft's own |
 | Rasterize | Rasterize… (as a live effect) | 1/1 | — |
 | Stylize | Drop Shadow, Feather, Inner Glow, Outer Glow, Round Corners, Scribble | 6/6 | Scribble Options' Settings presets |
-| SVG Filters | Apply SVG Filter… and the filter list | 1/1 | The submenu lists 14 default filters and then the document's own; the public page names no default set, so the set, names and values are VectorCraft's own, and a default is copied into the document when it is applied or edited. A filter that is the object's last effect is written to SVG as a `<filter>` element; otherwise SVG export draws the object as an image. A filter on a turned, skewed or unevenly scaled object in an imported SVG is kept and runs in that object's user space, and SVG export writes it back under the same transform |
+| SVG Filters | Apply SVG Filter… and the filter list | 1/1 | The submenu lists 14 default filters and then the document's own; the public page names no default set, so the set, names and values are VectorCraft's own, and a default is copied into the document when it is applied or edited. A filter that is the object's last effect is written to SVG as a `<filter>` element; otherwise SVG export draws the object as an image. A filter on a turned, skewed or unevenly scaled object in an imported SVG is kept and runs in that object's user space, and SVG export writes it back under the same transform. A blur or drop shadow that blurs more in one direction than another in the document (two different standard deviations, or one in a skewed or unevenly scaled user space) is kept as this effect rather than opened as an even Gaussian Blur or Drop Shadow |
 | Warp | Arc, Arc Lower, Arc Upper, Arch, Bulge, Shell Lower, Shell Upper, Flag, Wave, Fish, Rise, Fisheye, Inflate, Squeeze, Twist | 15/15 | warp point editing in envelopes; each style bends the bounds as one Bézier patch (`version` 1), so Arc's edges are cubic curves close to concentric arcs; warps saved without a version keep the earlier map, where Arc is a circular arc in the object's own proportions (#1578) |
 
 Document Raster Effects Settings exists. When the 64 million pixel cap on an effect image lowers