[Feature] Expose Bezier control points via FPDFPathSegment_GetBezierControlPoints

[Bill Denney]

PDFium's path segment readout exposes only the destination point
of each segment via FPDFPathSegment_GetPoint, even for
FPDF_SEGMENT_BEZIERTO. The two control points stored internally on
CFX_Path::Point are not retrievable through any public C API, so
embedders cannot reconstruct cubic Bezier curves read from existing
PDFs.

The constructor side already exposes all six floats via
FPDFPath_BezierTo, so the asymmetry is purely on the readout side.
Proposed addition, mirroring the existing shape:

  FPDF_EXPORT FPDF_BOOL FPDF_CALLCONV
  FPDFPathSegment_GetBezierControlPoints(FPDF_PATHSEGMENT segment,
                                          float* cp1_x, float* cp1_y,
                                          float* cp2_x, float* cp2_y);

Behavior:
  - Returns TRUE on success.
  - Returns FALSE without modifying out-params when segment type is
    not FPDF_SEGMENT_BEZIERTO.

Use case: every cross-language PDFium wrapper hits this limitation
when reading existing PDFs. pdfium-render's maintainer documented
the issue at https://github.com/ajrcarey/pdfium-render/issues/55
and the dedicated upstream-handoff issue
https://github.com/ajrcarey/pdfium-render/issues/99 was closed
2023-08-03 specifically asking for the upstream change. pypdfium2,
pdfium-rs, and an R binding (pdfium / github.com/humanpred/rpdfium)
all currently document the limitation and lose curve geometry on
the read path.

Alternatives considered:
  - Exposing the raw page content stream (e.g.
    `FPDFPage_GetRawContents`) would also unblock this, but is a
    much larger surface change with broader downstream impact.
  - Embedders cannot recover control points geometrically - many
    distinct curves share the same pair of endpoints.

Happy to draft the patch via Gerrit if there's interest.

[Lei Zhang]

Hi Bill,

Thanks for bringing this up. This makes sense and sounds
straightforward to implement, so please give it a try.

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