Fix converting wxBitmap to wxImage (wxGTK2)

Since f7247086c2 ("Fix storing wxBitmap data in GdkPixbuf", 2019-09-18),
919a4ec702 ("Fix drawing wxBitmap with mask", 2019-09-18) and other
commits (see #18498, #18508) RGBA wxBitmaps with masks are drawn properly
under wxGTK2 so only wxBitmap raw RGBA data should be transferred to
wxImage RGBA data because mask is stored in the target wxImage separately.

tests/graphics/bitmap.cpp

@@ -318,10 +318,6 @@ TEST_CASE("BitmapTestCase::ToImage", "[bitmap][image][convertto]")
         }
     }
 
-#if defined(__WXGTK20__) && !defined(__WXGTK3__)
-    // Bitmaps with both alpha and a mask don't work well in wxGTK2 so skip the test in this case.
-    WARN("Skipping test known not to work in wxGTK2.");
-#else
     SECTION("RGBA bitmap with mask")
     {
         // RGBA Bitmap
@@ -436,7 +432,6 @@ TEST_CASE("BitmapTestCase::ToImage", "[bitmap][image][convertto]")
         }
         CHECK(unmaskedPixelsCount == numUnmaskedPixels);
     }
-#endif // !__WXGTK20__
 }
 
 TEST_CASE("BitmapTestCase::FromImage", "[bitmap][image][convertfrom]")