LibGfx/JBIG2Writer+Tests/LibGfx: Refinement of symbol strip refines

[nico]

This adds writer support for refinement of a symbol dictionary entry that is itself a strip refinement.

This case is a bit awkward for the writer: To write the refinement, we need the bitmap that's being refined. But since that's a strip refine, we don't have that bitmap, we only have the parts it consists of.

As #26411 explains, the approach of calling into the loader to do the strip decoding doesn't work here: We need the decoded strip refinement while writing the strip itself. So we'd have to send a partially encoded strip to the loader, which isn't something we can currently do.

But since strip refinements only use a limited subset of full text strips (always a background color of 0, always a reference corner of topleft, always a composition operator of "or"), reimplementing the loader's logic for this case, while conceptually not nice, is not a lot of code. So let's just do that.

Using this, also add a test for this feature. I believe this tests the last missing refinement case :^)

The test uses regular symbols for the eyes (symbols 0 and 1) and the mouth (symbol 6).

The nose, 100x100 pixels, is conceptually split into 60x60, 40x60, 60x40, 40x40 tiles. Since symbols are sorted by increasing height, the latter two are symbols 2 and 3, the former are symbols 4 and 5.

Symbol 3 isn't actually the bottom right part of the nose, but contains the pixel data of an eye. This is for testing refinement below.

To test strip background color filling, the 60x60 tile is actually 59x60 pixels. And to test refinement, the top left tile is actually solid white instead of the top left part of the nose.

The first refinement symbol (id 7) refines the top left tile to a 59x60 tile that's the top left corner of the nose, but shifted several pixels to the right.

The second refinement symbol (id 8) is a strip refinement. It puts symbols 7 and 5 next to each other, to produce the top half of the nose (with the left part being shifted to the side).

The third refinement symbol (id 9) is a simple refinement of symbol 8 refines it to the actual top of the nose (to fix the shift, and to test refinement of strips in a symbol dictionary).

The fourth refinement symbol (id 10) is a strip refinement that puts symbol 9 in the first strip, with an offset of 1, and symbols 2 and 3 in a second strip. This means symbol 10 is almost the nose, but its bottom right contains the pixel data of an eye, since that's what symbol 3 contained.

Finally, the text region refines symbol 10 to the actual nose pixels, to test refinement of strips from a text region.

Whew!

Since the text region refines to the nose, it's useful to locally remove that refinement to test that the reference bitmap looks as expected. Similarly, it's useful to locally make the final strip refinement put symbol 8 instead of symbol 9 in the first strip, to make sure the input to symbol 9 looks as expected.

The test file decodes fine in all viewers I've tried (PDFium, Preview.app, pdf.js, mutool). (This case is much more awkward for the writer than for the loader, after all.)

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