Implement TASImage::Paint() for PDF

[vvolkl]

This Pull request:

Changes or fixes:

Previously TImage/TASImage only rendered to raster outputs: drawing an image and saving to PDF produced nothing but a Warning in <TASImage::Paint>: PDF not implemented yet.
TPDF::CellArrayBegin/CellArrayFill/CellArrayEnd were warning-only stubs and TASImage::Paint bailed out early for the PDF case. This changeset implements bitmap rendering to PDF. The early-return branch in TASImage::Paint is removed so PDF output flows through the same CellArray* path already used for PostScript, and TPDF now implements that API:
pixels accumulated by CellArrayFill are Flate-compressed and emitted as a proper PDF image XObject, the page's /Resources carries an /XObject name dictionary, and the page content
-stream only references the bitmap with a placement matrix and a /ImN Do operator. A new fixed object number is reserved for the shared image dictionary, shifting the first page object
from 51 to 52, and the XObjects are allocated and written at Close() since a PDF object cannot be opened while a page content stream is still in progress. A gtest under
graf2d/gpad/test renders a four-colour synthetic TImage to PDF and verifies both the XObject structure and that the embedded pixels decode back to the colours drawn. Verified on
single-page and multi-page documents with the cross-reference table validated; SVG output, which already worked, is unaffected.

Checklist:

• tested changes locally
• updated the docs (if necessary) (I don't think any docs referenced this functionality)

This PR fixes #22251

Assisted-by: Claude:claude-4.7-opus

[vvolkl]

Note that since the indices are changed any reference pdfs need to be updated. I think there are some in roottest but not sure if they are still used/up to date?

[github-actions]

Test Results

    22 files      22 suites   3d 10h 50m 36s ⏱️
 3 865 tests  3 865 ✅ 0 💤 0 ❌
76 256 runs  76 256 ✅ 0 💤 0 ❌

Results for commit a7c1cfd.

♻️ This comment has been updated with latest results.

[linev]

Few weeks ago I was asking myself - why it is not implemented?
And now it is there! Grate job!

I tested on my node - it works as expected.

I let Olivier @couet approve and merge PR - once he is back.

[linev]

I rebase pr once again - because of other changes in stressGraphics

[linev]

@vvolkl

Thanks again.
And I really like green ROOT CI :)