add pre-rendering in pdf/djvu readers

"HintPage" tasks are scheduled into the execution stack of UIManager
after each page drawing. These tasks should be checked immediately after
screen is refreshed so tasks are double-checked in the main loop.

frontend/ui/reader/readerview.lua

@@ -50,6 +50,7 @@ function ReaderView:paintTo(bb, x, y)
 			self.state.zoom,
 			self.state.rotation,
 			self.render_mode)
+		UIManager:scheduleIn(0, function() self.ui:handleEvent(Event:new("HintPage")) end)
 	else
 		if self.view_mode == "page" then
 			self.ui.document:drawCurrentViewByPage(
@@ -162,6 +163,10 @@ function ReaderView:onRotationUpdate(rotation)
 	self:recalculate()
 end
 
+function ReaderView:onHintPage()
+	self.ui.document:hintPage(self.state.page+1, self.state.zoom, self.state.rotation)
+end
+
 function ReaderView:onCloseDocument()
 	self.ui.doc_settings:saveSetting("render_mode", self.render_mode)
 end

frontend/ui/ui.lua

@@ -106,12 +106,7 @@ function UIManager:sendEvent(event)
 	end
 end
 
--- this is the main loop of the UI controller
--- it is intended to manage input events and delegate
--- them to dialogs
-function UIManager:run()
-	self._running = true
-	while self._running do
+function UIManager:checkTasks()
 	local now = { util.gettime() }
 
 	-- check if we have timed events in our queue and search next one
@@ -141,6 +136,17 @@ function UIManager:run()
 			end
 		end
 	until all_tasks_checked
+	return wait_until
+end
+
+-- this is the main loop of the UI controller
+-- it is intended to manage input events and delegate
+-- them to dialogs
+function UIManager:run()
+	self._running = true
+	while self._running do
+		local now = { util.gettime() }
+		local wait_until = self:checkTasks()
 		
 		--DEBUG("---------------------------------------------------")
 		--DEBUG("exec stack", self._execution_stack)
@@ -176,6 +182,8 @@ function UIManager:run()
 			self.refresh_type = 1
 		end
 		
+		self:checkTasks()
+		
 		-- wait for next event
 		-- note that we will skip that if in the meantime we have tasks that are ready to run
 		local input_event = nil