Add wxBitmap(wxImage, wxDC) ctor to all ports

This ctor was previously present only in wxMSW, make it available in all
ports to allow the same code to compile everywhere.

In most of them wxDC argument is simply ignored, but in wxGTK and wxOSX
it is used to assign the appropriate scale factor for the new bitmap.

Enable previously wxMSW-only unit test checking for this.

tests/graphics/bitmap.cpp

@@ -1699,12 +1699,9 @@ TEST_CASE("Bitmap::DC", "[bitmap][dc]")
     wxBitmap bmp(10, 10, dc);
     CHECK( bmp.IsOk() );
 
-    // wxBitmap ctor from wxImage and wxDC is not available in the other ports.
-#ifdef __WXMSW__
     wxImage image(10, 10);
     wxBitmap bmpFromImage(image, dc);
     CHECK( bmpFromImage.IsOk() );
-#endif // __WXMSW__
 #endif // wxUSE_SVG
 }
 
@@ -1727,7 +1724,6 @@ TEST_CASE("Bitmap::ScaleFactor", "[bitmap][dc][scale]")
     CHECK( bmp2.GetScaleFactor() == 2 );
     CHECK( bmp2.GetSize() == wxSize(8, 8) );
 
-#ifdef __WXMSW__
     // A compatible bitmap created from wxImage and this DC should also inherit
     // the same scale factor, but its size should be still the same as that of
     // the image.
@@ -1735,7 +1731,6 @@ TEST_CASE("Bitmap::ScaleFactor", "[bitmap][dc][scale]")
     wxBitmap bmp3(img, dc);
     CHECK( bmp3.GetScaleFactor() == 2 );
     CHECK( bmp3.GetSize() == wxSize(16, 16) );
-#endif // __WXMSW__
 }
 
 #endif // ports with scaled bitmaps support