Fix changeFontSize after added granularity (#7689)

Whith the new font size step of 0.5 (46a2d9c), the gesture
for increasing or decreasing font size would change the
size by only half of the given value.

frontend/apps/reader/modules/readerfont.lua

@@ -196,7 +196,7 @@ end
     UpdatePos event is used to tell ReaderRolling to update pos.
 --]]
 function ReaderFont:onChangeSize(direction, font_delta)
-    local delta = direction == "decrease" and -0.5 or 0.5
+    local delta = direction == "decrease" and -1 or 1
     if font_delta then
         self.font_size = self.font_size + font_delta * delta
     else