FR: smart image loader

[mangtronix]

"Smart" image loader that prioritizes loading the most immediately useful (to the user) images and reduces experienced latency when navigating through the book.

[mangtronix (mang)]

We currently preload some page images in 1up and 2up mode to reduce the latency when the user flips the page. We could be smarter about only starting to preload the out of view images once the in-view ones are loaded.

Thumbnail mode currently triggers the load of many images. This can cause a large delay before the visible images are loaded when scrolling in thumbnail mode or switching from thumbnail to 1up or 2up. For thumbnail mode it would make sense to only load n images at a time, with priority to the lowest numbered ones completely inside the view.

[mangtronix (mang)]

Thumbnail images are lazy loaded. At most 4 (number is configurable) thumbnail images should be loading at once.

[mangtronix (mang)]

In thumbnail mode there should never be more than 4 thumbnails loading at once. If you scroll the page view to a new location (or e.g. type in a new page in the page number box) there should be a minimal delay before the thumbnails in view load.

When switching from thumbnail to the other modes, there should be not too long of a delay before the page images load. The problem before was that many thumbnails could be queued for load and new page images (e.g. after switching to 1up) would not start downloading until the thumbnails had finished.