Fix crash when changing pages fast on presentation mode

The crash would happen because we were setting partial updates to true on requests
that were synchronous, because that sync request was cancelling an async one (prerender)
so now we only set newRequest partial updates to true when cancellingm another one if it's asynchronous

And a sync but with partial updates request would make stuff crash because the request
would all be processed on the main thread but generator_pdf.cpp cakks signalPartialPixmapRequest
with a queued connection so we that function was called the request would have already gone