fix(optimize): make pictures under a stencil mask smaller (#5847)

* fix(optimize): make pictures under a stencil mask smaller

An image with any /Mask was skipped by the colour and grey settings. That is
right for a colour-key mask, which names sample values that resampling and JPEG
would change. A stencil mask is an image of its own, with its own resolution
(ISO 32000-2 8.9.6.3), so the picture can change under it and the mask stays as
it is. Some exporters put one on every picture, even where it hides nothing, so
whole drawing sets of pictures were left at full size.

A mask that can't be read as a stencil, such as a reference to nothing, still
leaves its picture alone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(optimize): only a mask that says /ImageMask true counts as a stencil

A stream /Mask without /ImageMask true let its picture be replaced, although a
mask that isn't a stencil is meant to leave its picture alone. It now does.

The stencil test also checks that the hole still shows what's behind it. The first
pixel all round is left out, because the coarser picture blends into it a little
(at most 26 of 255 in this test).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* test(optimize): draw a coloured page behind the stencil test's picture

On a white page, a hole painted white looks the same as an open hole, so the check
on the hole couldn't tell them apart.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: quincy-jc <184324679+quincy-jc@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

parity/acrobat-features.toml

@@ -7373,8 +7373,8 @@ milestone = "M11"
 status = "shipped"
 commands = ["optimize.reduce"]
 tools = ["doc_reduce"]
-evidence = ["crates/ui-egui/tests/it/create.rs::reduce_file_size_writes_a_compact_copy", "crates/ui-egui/tests/it/create.rs::reduce_file_size_saves_nothing_when_the_file_is_already_small", "crates/engine/src/tests.rs::create_and_reduce", "crates/automation/tests/it/automation.rs::creating_and_reducing_through_tools", "crates/automation/tests/it/automation.rs::optimizing_through_tools", "crates/optimize/tests/optimize.rs::an_identity_decode_array_does_not_stop_reduce"]
-notes = "images above 225 ppi to 150 ppi JPEG medium (after merging page pictures printed as one-pixel strips into one image), black-and-white images averaged down to 300 ppi above 450 ppi, and stored as CCITT Group 4 whenever that is smaller, thumbnails dropped, resources merged, object streams; scanner images with the default decode array are included; a copy that isn't smaller is not saved (\"already as small as it can be made\"); runs in the background with a progress bar and Cancel"
+evidence = ["crates/ui-egui/tests/it/create.rs::reduce_file_size_writes_a_compact_copy", "crates/optimize/tests/optimize.rs::a_picture_with_a_stencil_mask_is_downsampled_under_its_mask", "crates/optimize/tests/optimize.rs::a_picture_whose_mask_is_not_a_stencil_is_left_alone", "crates/ui-egui/tests/it/create.rs::reduce_file_size_saves_nothing_when_the_file_is_already_small", "crates/engine/src/tests.rs::create_and_reduce", "crates/automation/tests/it/automation.rs::creating_and_reducing_through_tools", "crates/automation/tests/it/automation.rs::optimizing_through_tools", "crates/optimize/tests/optimize.rs::an_identity_decode_array_does_not_stop_reduce"]
+notes = "images above 225 ppi to 150 ppi JPEG medium (after merging page pictures printed as one-pixel strips into one image), black-and-white images averaged down to 300 ppi above 450 ppi, and stored as CCITT Group 4 whenever that is smaller, thumbnails dropped, resources merged, object streams; scanner images with the default decode array are included; pictures under a stencil mask are included and the mask stays as it is, though samples it hides still count when resampling, so their colour can reach its edge; pictures with a colour-key mask are left alone; a copy that isn't smaller is not saved (\"already as small as it can be made\"); runs in the background with a progress bar and Cancel"
 
 [[feature]]
 id = "optimize.compatibility-level"