remove page widget surface on size request, not on size allocate

[jchilds0]

Document widget uses size allocate to position the page widgets, which conflicts with the previous use of size allocate to invalidate the surface cache.

This should resolve:

• Flashing since last update #876 ,
• Transparent background flickers when scrolling recolored document #877 ,
• CJK characters rendering regression #879 ,
• Transparent background flickers when scrolling recolored document #877.
• lagging when scrolled when recolor is set #886 .

Transparency doesn't really work on my machine, and its fast enough that I didn't have lag scrolling so it would be good to have people that can reproduce those issue test.

zathura_page_widget_update_surface should only be called from the render thread, is this also the case for where I've changed gtk_widget_set_size_request to zathura_page_widget_set_size_request? If not, whats the correct way to queue this function to be called in the render thread at some point?