Page browser: show nightmode thumbnails when in nightmode (#11091)

Just prevent page thumbnails ImageWidgets to be nightmode
inverted, unlike all other ones which are expected to be
double inverted to get their original colors shown.
The same thumbnail can be used and cached in both day and
night modes, unless "nightmode_images" is enabled and have
crengine itself invert images, making thumbnails different.

frontend/apps/reader/modules/readerthumbnail.lua

@@ -250,6 +250,10 @@ end
 function ReaderThumbnail:getPageThumbnail(page, width, height, batch_id, when_generated_callback)
     self:setupCache()
     self.current_target_size_tag = string.format("w%d_h%d", width, height)
+    if self.ui.rolling and Screen.night_mode and self.ui.document.configurable.nightmode_images == 1 then
+        -- We'll get a different bb in this case: it needs its own cache hash
+        self.current_target_size_tag = self.current_target_size_tag .. "_nm"
+    end
     local hash = string.format("p%d-%s", page, self.current_target_size_tag)
     local tile = self.tile_cache and self.tile_cache:check(hash)
     if tile then
@@ -431,7 +435,7 @@ function ReaderThumbnail:_getPageImage(page)
             self.ui.document:setGammaIndex(30) -- as downscaling will make text grayer
         end
         self.ui.document:setImageScaling(false) -- No need for smooth scaling as all will be downscaled
-        self.ui.document:setNightmodeImages(false) -- We don't invert page images even if nightmode set: keep images as-is
+        -- (We keep "nighmode_images" as it was set: we may get and cache a different bb whether nightmode is on or off)
         self.ui.view.state.page = page -- Be on requested page
         self.ui.document:gotoPage(page) -- Current xpointer needs to be updated for some of what follows
         self.ui.bookmark:onPageUpdate(page) -- Update dogear state for this page