-nf drops CropBox: normalized render falls back to MediaBox dimensions

[valandi]

PdfFontNormalizer.normalize() copies only the MediaBox to the normalized
page. A source page whose CropBox is tighter than its MediaBox renders at
CropBox dimensions normally, but at MediaBox dimensions under -nf - the
checkpoint changes size and every baseline mismatches.

Pinned by @ignore'd tests in PdfFontNormalizerEdgeCaseTest
(normalized_page_preserves_cropbox_dimensions,
normalized_render_matches_cropped_dimensions).

Found by code reading during the -nf test-corpus work; no CropBox!=MediaBox
page occurred in the 28-file real-PDF sample.