feat(svm): draw the bitmaps a metafile carries, as png where that is smaller

[andiwand]

🤖 Generated with Claude Code

Closes #194. Stage 4 of #772, stacked on #785 → #784 → #779 — review those
first; this branch's base is feat/svm-text.

The empty frame

odr-private/svm/Vyplaty.svm is a 1.63 MB file, and 1.59 MB of it is a single
BMPEXSCALE: the whole data area of the chart is one bitmap. We drew the axes,
the grid and 79 labels around nothing. It is the only bitmap in 1125 harvested
metafiles, and it is exactly the file #194 named.

No decoder was needed

SvmReader reads a dib with ReadDIB(…, bFileHeader=true), so the action body
holds it with its BITMAPFILEHEADER — the bytes already are a .bmp file
and a browser reads them as one. The only thing to work out is the length, and
bfSize is no help: ImplWriteDIBFileHeader computes it from the
uncompressed size, so for a compressed dib it lies (this file's mask claims
522214 bytes and occupies 29478). The header's own biSizeImage — or width,
height and bit count — is the answer.

png, because 2.9 MB per page is not a rendering

The bytes go out as a png rather than a bmp wherever the pixels can be copied
row by row: uncompressed, 1/4/8/24/32 bits, palette expanded, rows turned right
way up, bgr swapped. Re-packing, not decoding.

	bmp	png
the bitmap	1.59 MB	33 KB
Vyplaty.ods/sheet1.html	2.9 MB	158 KB

Anything compressed (RLE, bit fields) still goes out as the bmp it is —
browsers read those, verified in Chrome for the RLE8 mask this very file
carries.

The png writing pdf_image.cpp already had moves to util::png::write so both
callers can reach it, with its two tests.

The mask

A BMPEX may carry a transparency mask: a second dib, white where the bitmap
does not show (Bitmap::Bitmap(rBmp, rMask) inverts it into an alpha
channel). An svg <mask> keeps what is white, so the mask image goes through
an inverting feColorMatrix, written once per document.

Still open

Neither occurs in the corpus, both are noted in svm/PLAN.md: ZCOMPRESS, a
LibreOffice-only compression whose zlib stream would have to be inflated first
(miniz is already a dependency), and the MASK family, which stencils one
colour through a bitmap.

Verification

2 new tests, both from inline bytes — a dib built by hand in the test, drawn
where the action puts it and re-packed as a png; and a BMPEX whose mask
element, inverting filter and reference all come out.

Rendered in headless Chrome: the chart now matches what LibreOffice's own
--convert-to svg makes of the same file. The reference output moves for the
two pages of ods/Vyplaty.ods and nothing else; the pin is advanced in this
branch.