Only emit perc_changed signal when the percentage actually changed

QtWebEngine emits scrollPositionChanged a lot during smooth scrolling, and
there's no reason we need to update percentages when they didn't *actually*
change.

This reduces the updates with a single spacebar press from 6-7 to 2-3 on my
machine, which might not be enough though.

See #2233

doc/changelog.asciidoc

@@ -106,6 +106,7 @@ Fixes
 - The "try again" button on error pages works correctly again.
 - :spawn -u -d is now disallowed.
 - :spawn -d shows error messages correctly now.
+- Performance improvements for smooth scrolling
 
 v0.11.0
 -------

qutebrowser/browser/webengine/webenginetab.py

@@ -333,9 +333,10 @@ class WebEngineScroller(browsertab.AbstractScroller):
                 perc_y = min(100, round(100 / dy * jsret['px']['y']))
 
             self._at_bottom = math.ceil(jsret['px']['y']) >= dy
-            self._pos_perc = perc_x, perc_y
 
-            self.perc_changed.emit(*self._pos_perc)
+            if self._pos_perc != (perc_x, perc_y):
+                self._pos_perc = perc_x, perc_y
+                self.perc_changed.emit(*self._pos_perc)
 
         js_code = javascript.assemble('scroll', 'pos')
         self._tab.run_js_async(js_code, update_pos_cb)