wasm: Only show resize border if direct parent window enables it

A top level window with a window frame will have the has-border class,
and hence a resize border, but child windows of that window shouldn't
have their own resize borders.

Ideally we wouldn't generate these non-client-area elements for child
windows, but changing that requires more work, as it involves creating
and destroying these elements on the fly if a window goes from being
a child window to a top level and vice versa.

Pick-to: 6.8
Change-Id: I64ec27d87df952419284354b50420df8e48824d8
Reviewed-by: Piotr Wierciński <piotr.wiercinski@qt.io>

src/plugins/platforms/wasm/qwasmcssstyle.cpp

@@ -64,7 +64,7 @@ const char *Style = R"css(
 
 .qt-window.no-resize > .resize-outline { display: none; }
 
-.qt-window.has-border:not(.maximized):not(.no-resize) .resize-outline {
+.qt-window.has-border:not(.maximized):not(.no-resize) > .resize-outline {
     display: block;
 }