Redraw current page if necessary on pressing Back

For PDF and DjVu files (but not for crereader) the command handler for
"Back" key can call addJump() in order to avoid losing the top of the
jump history on return. This (addJump()) can cause the TOC to be
retrieved and the message "Retrieving TOC..." to be displayed.
If this happens then we need to redraw the current page after displaying
our own "Already first jump!" message.

unireader.lua

@@ -2261,7 +2261,13 @@ function UniReader:addAllCommands()
 		"go backward in jump history",
 		function(unireader)
 			local prev_jump_no = 0
+			local need_refresh = false
 			if unireader.jump_history.cur > #unireader.jump_history then
+				-- addJump() will cause a "Retrieving TOC..." msg, so we'll
+				-- need to redraw the page after our own
+				-- ifo msg "Already first jump!" below
+				if not self.toc then need_refresh = true end
+
 				-- if cur points to head, put current page in history
 				unireader:addJump(self.pageno)
 				prev_jump_no = unireader.jump_history.cur - 2
@@ -2274,6 +2280,9 @@ function UniReader:addAllCommands()
 				unireader:goto(unireader.jump_history[prev_jump_no].page, true)
 			else
 				showInfoMsgWithDelay("Already first jump!", 2000, 1)
+				if need_refresh then
+					unireader:redrawCurrentPage()
+				end
 			end
 		end)
 	self.commands:add(KEY_BACK,MOD_SHIFT,"Back",