Fix re-rendering of images when using partial updates
Partial updates trigger when the page is taking "too much" to render (> 500 ms).
When this happens we store a pixmap for the page, this meant that Page::hasPixmap returned true, so when moving the viewport around we would think that that page was already rendered, and thus needed no rendering so we didn't add it to the list of requested pixmaps.
Then on document when seeing the new list of requested pixmaps we would go and check and say "oh there's one request going on that we don't want anymore, let's cancel it", so we would cancel the page that we actually wanted (and clear the partial pixmap)
Then on the next scroll we would realize we did not have that page pixmap and then request it again which since it's the current page would immediately stop all other renders and start this one.
Then we would get a temporary pixmap and the loop of cancellations and requesting again would keep happening as long as the user moved the viewport.
We fix that my making hasPixmap return false if the pixmap we have is a partial one, because that's what the function meant "forever" until we recently introduced partial updates so all the calls to hasPixmap actually mean "hasNonPartialPixmap"