ensure path icon is centered.

src/Avalonia.Themes.Default/PathIcon.xaml

@@ -9,7 +9,8 @@
         <Viewbox Height="{TemplateBinding Height}"
                  Width="{TemplateBinding Width}">
           <Path Fill="{TemplateBinding Foreground}"
-                Data="{TemplateBinding Data}" />
+                Data="{TemplateBinding Data}"
+                Stretch="Uniform" />
         </Viewbox>
       </ControlTemplate>
     </Setter>

src/Avalonia.Themes.Fluent/PathIcon.xaml

@@ -17,7 +17,8 @@
         <Viewbox Height="{TemplateBinding Height}"
                  Width="{TemplateBinding Width}">
           <Path Fill="{TemplateBinding Foreground}"
-                Data="{TemplateBinding Data}" />
+                Data="{TemplateBinding Data}"
+                Stretch="Uniform" />
         </Viewbox>
       </ControlTemplate>
     </Setter>