core: reserve vector sizes as much as we can (#9118)

avoid reallocations as much as possible with a few edge cases where the
reservation overshoots a tiny bit. but a few bytes of memory short term
is better used then the overhead of potential reallocation.

src/render/decorations/DecorationPositioner.cpp

@@ -122,6 +122,9 @@ void CDecorationPositioner::onWindowUpdate(PHLWINDOW pWindow) {
 
     //
     std::vector<CDecorationPositioner::SWindowPositioningData*> datas;
+    // reserve to avoid reallocations
+    datas.reserve(pWindow->m_dWindowDecorations.size());
+
     for (auto const& wd : pWindow->m_dWindowDecorations) {
         datas.push_back(getDataFor(wd.get(), pWindow));
     }