For valid PDFs, this makes no difference.
For invalid PDFs, we now assert during the cast in resolve_to() instead
of returning a PDFError. However, most PDFs are valid, and even for
invalid PDFs, we'd previously keep the old color space around when
getting the PDF error and then usually assert later when the old
color space got passed a color with an unexpected number of components
(since the components were for the new color space).
Doesn't affect any of the > 2000 PDFs I use for testing locally,
is less code, and should make for less surprising asserts when it
does happen.
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