Multi-Select + Tables: fixed an issue which could lead to an extra vertical offset in the Header row. (#8250, #7994)

Because BeginMultiSelect() does `ms->ScopeRectMin = window->DC.CursorMaxPos = window->DC.CursorPos` at a time where CursorPos is already past MaxPos.y because of ItemSpacing.y. Accumulate spacing.

docs/CHANGELOG.txt

@@ -61,6 +61,8 @@ Other Changes:
 - Multi-Select:
   - Fixed an issue using Multi-Select within a Table causing column width measurement to
     be invalid when trailing column contents is not submitted in the last row. (#9341, #8250)
+  - Fixed an issue using Multi-Select within a Table with the right-most column visible,
+    which could lead to an extra vertical offset in the Header row. (#8250)
   - Box-Select: fixed an issue using ImGuiMultiSelectFlags_BoxSelect1d mode while scrolling.
     Notably, using mouse wheel while holding a box-selection could lead items close to windows
     edges from not being correctly unselected. (#7994, #8250, #7821, #7850, #7970)