RenderPageProgressive method

Cancellable variant of RenderPage that drives PDFium's progressive rendering loop and polls an IPdfCancellationToken between iterations so long high-DPI renders can abort mid-page instead of blocking until completion.

RenderPageProgressive renders the current page into the supplied Bitmap using PDFium's FPDF_RenderPageBitmap_Start + FPDF_RenderPage_Continue loop. Between iterations, PDFium's IFSDK_PAUSE callback polls AToken.IsCancelled — when the application has cancelled, the loop unwinds cleanly and the bitmap holds the partial work done so far.

The same zero-copy fast path introduced in v1.21.0 applies: PDFium renders directly into the destination TBitmap's DIB buffer instead of staging through an internal buffer. A legacy per-scanline fallback exists for the unlikely case where the destination DIB cannot be wrapped.

The return value identifies how the render finished:

• prsDone — the bitmap is fully rendered. Equivalent to a successful synchronous RenderPage.
• prsCancelled — the caller cancelled mid-loop. The bitmap holds the partial result up to the iteration where cancellation was detected; treat it as best-effort preview content.
• prsFailed — PDFium reported FPDF_RENDER_FAILED. The bitmap state is undefined.

Pass AToken = nil to render to completion without ever pausing. In that case the function still pumps FPDF_RenderPage_Continue until DONE but never returns prsCancelled. This is useful when the caller wants the progressive loop's intermediate redraw points (a custom OnPaint that mirrors the in-progress bitmap) but does not need cancellation.

• The bitmap must be allocated by the caller and have PixelFormat = pf32bit for the zero-copy path to engage; other pixel formats are upgraded to pf32bit before the render starts.
• Width / Height are in pixels (the destination bitmap's dimensions). The page is scaled to fit those dimensions before the progressive loop kicks off.
• RenderPageProgressive itself is synchronous from the caller's point of view — it returns when the render finishes or cancels. To free the main thread, drive RenderPageProgressive from a worker thread. TPdf serialises render calls per instance; callers still must avoid mutating or unloading the document while a render is in flight.
• An EPdfError is raised when Bitmap is nil — the contract requires a caller-allocated bitmap because the zero-copy path needs a known DIB layout.

RenderPage, RenderTile