[bobsingor]

Follow-up to #93. Layer mode (EPDF_EMBEDDER_LAYER_MODE=1) only made layers of files a test opens with OpenDocument(). Most font tests build their PDFs in memory or start from CreateEmptyDocument(), so they never ran on a layer with the read-only check and the layer DCHECKs on.

What changes

• Loaders that go through layer mode:
  • EmbedderTest::OpenDocumentFromMemory() opens bytes the way OpenDocument() opens a file;
  • embedpdf_test::LoadMemDocumentForTest() with ScopedTestDocument does the same for a document a test keeps itself, and reports what it left in the layer when it closes;
  • EmptyPdf() replaces CreateEmptyDocument() for tests that write into a new document;
  • LayerFixture::SaveDelta() saves a layer's changes for Reopen().
• Tests moved onto them:
  • the font tests (APlainDocumentIsWalkedWholeEachTime stays plain on purpose);
  • the two /ActualText tests;
  • the two rich-text last-resort tests.
• Two new layer tests:
  • AFontWrittenOnALayerReadsTheSameReopened: a font written on a layer, saved, then reopened from its delta lists, serves and resolves the same, and reading it copies nothing into the layer.
  • AWriteToAFontsWidthsGivesItANewDisplayProgram: a write to an existing font's /Widths shows a new display program in a list during the write; an abort brings the old one back and a commit keeps the new one.
• Fix in CPDF_DocumentFonts::Forget: a display program that only a changed font dictionary read stayed fetchable by its old hash. It is now dropped, so the hash finds nothing.
• Test fix: ExpectDisplayProgramDrawsTheKeptFont read the font dictionary through GetMutableRoot(), which copied the catalog into the layer. It now reads const, as the runtime's read path does.
• Writers list: the 14 tests that write on purpose are recorded in testing/embedpdf_layer_mode_writers.txt, including EPDFFormEmbedderTest.NewFieldAppearanceIsAFormXObject from Read, write and draw with a document's own fonts #93.
• Ledger: the layer-mode rule now says how an in-memory test document gets into layer mode.

Testing (macOS arm64, debug)

• 29 font, /ActualText and last-resort tests pass on plain documents.
• run_layer_mode.py over the 128 font-related tests: all pass, and every read-only font test leaves 0 objects in its layer.
• Full run_layer_mode.py over 1740 tests: no new layer-only failures.
  • Every test that fails only in layer mode matches testing/embedpdf_layer_mode_expectations.txt.
  • The one error is EPDFSignatureEmbedderTest.SignEncryptedDocument, which is local-only: its fixture is untracked, so the test skips on CI.
• Plain run: the same 30 known macOS pixel failures as on embedpdf/main.
• pdfium_unittests: 1312 pass.
• check-fork-tags.py --strict and check-c-api.py pass.

The Linux Docker suites and layer mode run in CI.