Add wxBitmap(size, dc) ctor to all ports

Previously this ctor was only available in wxMSW and wxOSX, which was
especially strange as the equivalent Create() overload, taking wxDC, was
already available in all ports.

include/wx/gtk1/bitmap.h

@@ -64,6 +64,7 @@ public:
     wxBitmap() {}
     wxBitmap( int width, int height, int depth = -1 ) { Create( width, height, depth ); }
     wxBitmap( const wxSize& sz, int depth = -1 ) { Create( sz, depth ); }
+    wxBitmap( int width, int height, const wxDC& dc ) { Create(width, height, dc); }
     wxBitmap( const char bits[], int width, int height, int depth = 1 );
     wxBitmap( const char* const* bits );
     wxBitmap( const wxString &filename, wxBitmapType type = wxBITMAP_DEFAULT_TYPE );