Effects: Glass loads its texture from an image file

Effect › Distort › Glass… offered only its four built-in textures. The
reference app's Texture menu also has Load Texture, which takes an
image file as the glass surface.

The Glass dialog now has a Load Texture… button. It reads a PNG, JPEG,
GIF, WebP, TIFF, BMP, PSD or PSB file within the pixel allowance the
renderer draws images in, and the Texture menu gains Loaded Texture.
The effect stores the image's gray levels in `textureImage` as a
grayscale PNG in base64, so the document saves the texture with the
effect, and copies of the effect carry it. effect.apply and
effect.setParams take `textureImage` as {path}, {name, dataBase64} or
the stored string; any other value is an error.

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. A Glass without `textureImage` draws and saves as before.

Docs: docs/mcp.md (Glass textures), docs/effects-parity.md,
docs/gaps.md, docs/target-app-parity.md.

crates/engine/src/cmd/fileio/mod.rs

@@ -60,7 +60,7 @@ pub(crate) use load::native_file;
 pub(crate) use load::source;
 pub use load::{
     Loaded, RasterImage, ReadFile, detect, file_name, load, load_with, open_bytes, open_bytes_with, open_read, open_template, raster_image,
-    read_for_open,
+    read_for_open, texture_image,
 };
 pub use load::{losses_summary, overwrite_losses};
 pub(crate) use native::preview_png;