On pressing Back crereader loses last position

This fix makes the behaviour of Back command handler in crereader
identical with that in unireader, i.e. the last position is saved in
Jump History when pressing Back, so you can return to it by pressing
Shift-Back enough times.
Also, with this change we would need to handle the need for redrawing
the current page because the :addJump() method can force the call to
fillToc() (if TOC is not already present) and this will display
"Retrieving TOC..." message which will need clearing away.

crereader.lua

@@ -636,12 +636,29 @@ function CREReader:adjustCreReaderCommands()
 	self.commands:add(KEY_BACK, nil, "Back",
 		"go backward in jump history",
 		function(self)
-			local prev_jump_no = self.jump_history.cur - 1
+			local prev_jump_no = 0
+			local need_refresh = false
+			if self.jump_history.cur > #self.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
+				self:addJump(self.doc:getXPointer())
+				prev_jump_no = self.jump_history.cur - 2
+			else
+				prev_jump_no = self.jump_history.cur - 1
+			end
+
 			if prev_jump_no >= 1 then
 				self.jump_history.cur = prev_jump_no
 				self:goto(self.jump_history[prev_jump_no].page, true, "xpointer")
 			else
 				showInfoMsgWithDelay("Already first jump!", 2000, 1)
+				if need_refresh then
+					self:redrawCurrentPage()
+				end
 			end
 		end
 	)