[Theme] Drop stale pixmap caches on palette-only preview (#7600)

A palette preview repaints widgets through applyStyleAndPalette without running
themeChangedSlot, so generator-owned pixmaps stayed at the previous palette
until the next full theme switch. Clear them where the palette is applied.

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

cockatrice/src/interface/theme_manager.cpp

@@ -558,6 +558,9 @@ void ThemeManager::applyStyleAndPalette(const QString &themeName,
         widget->update();
     }
 
+    // Palette-only previews do not run themeChangedSlot(), so drop snapshots
+    // generated under the previous palette here as well.
+    clearPixmapGeneratorCaches();
     emit paletteChanged();
 }