ReaderRolling: quicker partial rerenderings with EPUBs

Only available with EPUBs containing 2 or more fragments,
and a file size large enough to ensure a cache file is used.
The idea is simply, on any rendering setting change, to
skip the rerendering of the full book and to defer any
rerendering to the moment we draw a DocFragment, and
render only it.
So, on a setting change, only the fragment containing the
current page will be rerendered, and the new fragments we
may cross while turning pages.
When having done so, KOReader is in a degraded state (the
full page count is incorrect, the ToC is invalid...).
So, a full rerendering is needed, and one will happen
in the background, and when the user is idle, we reload
seamlessly and quickly from the cache file it has made.
ReaderFlipping will show some icons in the top left
corner to let it know at which steps in this procress
we are.

frontend/apps/reader/modules/readerthumbnail.lua

@@ -407,10 +407,12 @@ function ReaderThumbnail:_getPageImage(page)
     if self.ui.view.highlight.lighten_factor < 0.3 then
         self.ui.view.highlight.lighten_factor = 0.3 -- make lighten highlight a bit darker
     end
+    self.ui.highlight.select_mode = false -- Remove any select mode icon
 
     if self.ui.rolling then
         -- CRE documents: pages all have the aspect ratio of our screen (alt top status bar
         -- will be croped out after drawing), we will show them just as rendered.
+        self.ui.rolling.rendering_state = nil -- Remove any partial rerendering icon
         self.ui.view:onSetViewMode("page") -- Get out of scroll mode
         if self.ui.font.gamma_index < 30 then  -- Increase font gamma (if not already increased),
             self.ui.document:setGammaIndex(30) -- as downscaling will make text grayer
@@ -503,6 +505,7 @@ end
 
 -- CRE: emitted after a re-rendering
 ReaderThumbnail.onDocumentRerendered = ReaderThumbnail.resetCache
+ReaderThumbnail.onDocumentPartiallyRerendered = ReaderThumbnail.resetCache
 -- Emitted When adding/removing/updating bookmarks and highlights
 ReaderThumbnail.onBookmarkAdded = ReaderThumbnail.resetCachedPagesForBookmarks
 ReaderThumbnail.onBookmarkRemoved = ReaderThumbnail.resetCachedPagesForBookmarks