refactor

Expansion symbol rendering relied on wxDC logical raster ops
(wxOR / wxAND_INVERT) during buffer composition. On wxGTK this
produced incorrect symbol/background inversion, likely due to
backend differences in logical-function handling.

This replaces the logical blits in combineBuffers() with explicit
per-pixel compositing that preserves the intended bitwise behavior
while producing consistent results across platforms.

Tested:
- Linux: fixes incorrect expansion symbol rendering/export
- Windows: no regression observed in rendering/export

src/gui/about_window.cpp

@@ -40,8 +40,7 @@ const char* MSE_AUTHORS[] = {
   "Olivier Bocksberger (G-e-n-e-v-e-n-s-i-S)",
   "Brendan Hagan (haganbmj)",
   "Thomas Tkacz (TomTkacz)",
-  "CaiCai (247321453)",
+  "CaiCai (247321453)"
-  "Amy Markey (amyinspace)"
 };
 
 void AboutWindow::draw(DC& dc) {

src/gui/set/window.cpp

@@ -50,7 +50,7 @@ SetWindow::SetWindow(Window* parent, const SetP& set)
   SetIcon(load_resource_icon(_("app")));
 
   // avoid flicker
-  SetBackgroundStyle(wxBG_STYLE_SYSTEM);
+  SetBackgroundStyle(wxBG_STYLE_PAINT);
 
   // initialize menu bar
   auto menuBar = new wxMenuBar();