Part of #1286: constant alpha masks are drawn white, so the page shows where soft masks are read by luminosity

A CMYK page with transparency is drawn through an alpha soft mask that covers the page (a
non-isolated group). Its cover was black: opaque as alpha, but nothing shows in a reader that
takes the mask's luminosity. A white cover is the same mask either way.

crates/pdf/src/export.rs

@@ -605,10 +605,11 @@ fn cover(s: &mut Surface, page: Rect, level: u8, opacity: f32) {
 
 /// An alpha mask of constant `alpha` over the page. A group drawn through it is a transparency
 /// group of that opacity that, unlike the writer's opacity groups, is not isolated.
+/// The cover is white, so a reader that takes every soft mask as a luminosity mask shows the same.
 fn constant_mask(s: &mut Surface, page: Rect, alpha: f32) -> krilla::mask::Mask {
     let mut sb = s.stream_builder();
     let mut ms = sb.surface();
-    cover(&mut ms, page, 0, alpha);
+    cover(&mut ms, page, 255, alpha);
     ms.finish();
     krilla::mask::Mask::new(sb.finish(), krilla::mask::MaskType::Alpha)
 }

crates/pdf/src/tests_cmykblend.rs

@@ -76,3 +76,12 @@ fn soft_mask_groups_keep_screen_luminance() {
     // The page's group blends in CMYK; the mask's group measures luminance in RGB.
     assert!(t.contains("/CS/DeviceCMYK") && t.contains("/S/Luminosity") && t.contains("/CS/DeviceRGB"), "{t}");
 }
+
+#[test]
+fn constant_alpha_masks_are_white() {
+    // #1286: a black cover hides the whole page in readers that take the mask's luminosity.
+    for grouped in [true, false] {
+        let t = text(&doc(ColorMode::Cmyk, grouped));
+        assert!(t.contains("/S/Alpha") && t.contains("111rg") && !t.contains("000rg"), "grouped {grouped}: {t}");
+    }
+}