Defer warnings when PixBuf is not available.
Defer warnings when PixBuf is not available.

Previously, a missing PixBuf caused a warning at import-time.
Now the exception is saved and re-raised when an image needs
to be loaded: there is no warning if PixBuf is not used at all.

Kozea/WeasyPrint,Kozea/WeasyPrint,andrewleech/WeasyPrint,andrewleech/WeasyPrint,prepare/TestWeasyPrint,jasco/WeasyPrint,prepare/TestWeasyPrint,marclaporte/WeasyPrint,marclaporte/WeasyPrint,jasco/WeasyPrint