mod: record self.globalzoom_orig to help zoom in/out

see: https://github.com/hwhw/kindlepdfviewer/issues/24

pdfreader.lua

@@ -86,7 +86,11 @@ function PDFReader:setzoom(page)
 		self.pan_y = self.offset_y
 		self.pan_by_page = true
 	end
+
 	dc:setZoom(self.globalzoom)
+	-- record globalzoom for manual zoom in/out
+	self.globalzoom_orig = self.globalzoom
+
 	dc:setRotate(self.globalrotate);
 	dc:setOffset(self.offset_x, self.offset_y)
 	self.fullwidth, self.fullheight = page:getSize(dc)