outputchangeset: Round passed scale

Converting to wl_fixed is lossy. Especially for non-round values like 1.15.

This causes various visual glitches and off sizes.

Given we're using units of 120ths for the fractional scale protocol it makes sense
to fix the passed values for anything else over the wayland protocol.

BUG: 465850

Testing done:
ran a 1920 screen at 1.15 scale, got a logical size of 1670 which matches what it should be

src/wayland/outputmanagement_v2_interface.cpp

@@ -166,7 +166,12 @@ void OutputConfigurationV2Interface::kde_output_configuration_v2_scale(Resource
     if (invalid) {
         return;
     }
-    const qreal doubleScale = wl_fixed_to_double(scale);
+    qreal doubleScale = wl_fixed_to_double(scale);
+
+    // the fractional scaling protocol only speaks in unit of 120ths
+    // using the same scale throughout makes that simpler
+    // this also eliminates most loss from wl_fixed
+    doubleScale = std::round(doubleScale * 120) / 120;
 
     if (doubleScale <= 0) {
         qCWarning(KWIN_CORE) << "Requested to scale output device to" << doubleScale << ", but I can't do that.";