Mirror Modifier: Add offsets for mirrored UVs

The mirror modifier now has two fields that specify a -1 to 1 offset for
the U and V axes when mirroring their coordinates.

D1844 by @circuitfox

release/scripts/startup/bl_ui/properties_data_modifier.py

@@ -569,6 +569,14 @@ class DATA_PT_modifiers(ModifierButtonsPanel, Panel):
         col.prop(md, "use_mirror_u", text="U")
         col.prop(md, "use_mirror_v", text="V")
 
+        col = layout.column(align=True)
+
+        if md.use_mirror_u:
+            col.prop(md, "mirror_offset_u")
+
+        if md.use_mirror_v:
+            col.prop(md, "mirror_offset_v")
+
         col = layout.column()
 
         if md.use_mirror_merge is True: