make up/down levels same

I use addition and substraction instead of multipiciation with factor
to make zoom deterministic (and this, one zoom up too many can be undone
with simple zoom out which wasn't the case).

pdfreader.lua

@@ -361,17 +361,17 @@ function PDFReader:inputloop()
 				self.altmode = true
 			elseif ev.code == KEY_PGFWD or ev.code == KEY_LPGFWD then
 				if self.shiftmode then
-					self:setglobalzoom(self.globalzoom*1.2)
+					self:setglobalzoom(self.globalzoom+0.2)
 				elseif self.altmode then
-					self:setglobalzoom(self.globalzoom*1.1)
+					self:setglobalzoom(self.globalzoom+0.1)
 				else
 					self:goto(self.pageno + 1)
 				end
 			elseif ev.code == KEY_PGBCK or ev.code == KEY_LPGBCK then
 				if self.shiftmode then
-					self:setglobalzoom(self.globalzoom*0.8)
+					self:setglobalzoom(self.globalzoom-0.2)
 				elseif self.altmode then
-					self:setglobalzoom(self.globalzoom*0.9)
+					self:setglobalzoom(self.globalzoom-0.1)
 				else
 					self:goto(self.pageno - 1)
 				end