... which mysteriously causes the Export PNG to produce a non-blank
image again.

The cause is probably that DocumentToGraphicRenderer::renderToGraphic()
records a VCL MetaFile, and the ReadFraction/WriteFraction do only
32 bits, while the Fraction interface promises to handle "long".

(regression from 2ce0aede)

Change-Id: Ic4289a3157ea802c17f0bc24db94ea45eabd9728