Release the DC after GetDC in get_dpi_for_window and font enumeration (#15919)

get_dpi_for_window's pre-8.1 fallback and get_font_list_by_enumeration
both called GetDC without a matching ReleaseDC, leaking a GDI handle
each call. get_dpi_for_window runs on every mouse-move over the 3D
viewport, so the leak exhausts the per-process GDI handle limit and
hangs the app within minutes on Windows 7/8.

Co-authored-by: Fernando Marino <f.marino@rheagroup.com>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

src/libslic3r/Emboss.cpp

@@ -990,6 +990,7 @@ EmbossStyles Emboss::get_font_list_by_enumeration() {
     std::vector<std::wstring> font_names;
     EnumFontFamilies(hDC, (LPCTSTR) NULL, EnumFamCallBack,
                      (LPARAM) &font_names);
+    ReleaseDC(NULL, hDC);
 
     EmbossStyles font_list;
     for (const std::wstring &font_name : font_names) {

src/slic3r/GUI/GUI_Utils.cpp

@@ -247,7 +247,9 @@ int get_dpi_for_window(const wxWindow *window)
 
         const HDC hdc = GetDC(hwnd);
         if (hdc == NULL) { return DPI_DEFAULT; }
-        return GetDeviceCaps(hdc, LOGPIXELSX);
+        const int dpi = GetDeviceCaps(hdc, LOGPIXELSX);
+        ReleaseDC(hwnd, hdc); // GetDC's handle must be released, unlike GetWindowDC's
+        return dpi;
     }
 #elif defined __linux__
     // TODO