Part of #2558: a drag draws zoomed-in raster effects at a lower resolution until it ends

When zoomed in, an object with a texture, a blur or a shadow was slow
to drag. Filtered content whose on-screen area passes the 8.4 million
pixel raster budget keeps no raster, and a shadow or glow larger than
4 million pixels keeps none either, so every frame of the drag ran the
effects again over up to 8.4 million pixels. A drag that scales or
rotates an object runs them again at any zoom.

The canvas now renders a tool's drag with RenderOptions::dragging. In
such a frame the renderer draws filtered content within 1,000,000
pixels, at a lower resolution where it would need more, unless it can
draw the content from an earlier frame's raster. On the multithreaded
pipeline it does the same for drop shadows, outer glows and inner
glows. No raster made that way is kept. They count as moved rasters,
so a raster made over them is made again after the drag, and the
desktop canvas renders the frame after the drag on the render worker.
That frame renders them as a still frame does. A dialog's preview,
exports and printing render as before.

zoomed_drag_perf times a drag's frames at 100% to 6400% with and
without dragging.

Docs: docs/target-app-parity.md.

docs/target-app-parity.md

@@ -757,6 +757,8 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
 
 - **Smart Blur modes:** Effect › Blur › Smart Blur… has a Mode menu with Normal, Edge Only and Overlay Edge (`blurMode`: `normal`, `edgeOnly` or `overlayEdge`; an effect saved without it draws as Normal). Edge Only draws the edges in white on black, and Overlay Edge draws them in white over the Normal blur. A pixel is on an edge when one of the pixels the search reads within Radius of it differs from it by more than Threshold in a channel and its channels, alpha included, add up to at least that pixel's. The search reads neighbors as the Normal blur does, kept within Radius, and always the four nearest pixels, so a color step gives a line about as wide as Radius, and at least one pixel wide, on the side with the larger sum (lines on both sides when the sums are equal). The white and the black keep each pixel's coverage. Behavior reference: [Summary of effects](https://helpx.adobe.com/illustrator/using/summary-effects.html).
 
+- **Dragging objects with raster effects when zoomed in (#2558):** during a drag with a tool on the canvas, the renderer draws filtered content (Gaussian Blur, Feather and the Photoshop-style filters) within 1,000,000 pixels, at a lower resolution where it would need more, unless it can draw the content from an earlier frame's raster. In the desktop app it does the same for drop shadows, outer glows and inner glows. No raster made that way is kept. The frame after the drag renders them as a still frame does, and a dialog's preview, exports and printing render as before. `zoomed_drag_perf` times a drag's frames at 100% to 6400%.
+
 - **Moving objects with raster effects (#1257):** during a drag or a dialog's preview, an object whose content only moved draws its cached drop shadows, outer glows and filtered content (Gaussian Blur, Feather and the Photoshop-style filters) from the rasters of an earlier frame at the nearest whole pixel, without running the effects again. The renderer first draws the moved content where the old content lies in its raster and compares the pixels. De-Interlace runs again. The frame after the drag runs the effects again and draws what a fresh renderer draws; on the desktop, the canvas renders that frame on the render worker. `texture_drag_perf` times a drag's frames and the frame after it.
 
 - **Glass's Load Texture:** Effect › Distort › Glass… has a Load Texture… button that reads a PNG, JPEG, GIF, WebP, TIFF, BMP, PSD or PSB file and adds Loaded Texture to the Texture menu (`textureImage`: `{path}` or `{name?, dataBase64}`, stored in the effect and saved with the document as a grayscale PNG in base64). The texture lies one image pixel per point at Scaling 100%, centered on the object and repeated; each pixel's luma is the surface's height, from 0 for black to 1 pt for white, and a transparent pixel counts as middle gray. Scaling and Invert act on it as on the built-in textures. A Glass saved without `textureImage` draws and saves as before.