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.

src/qt/bitmap.cpp

@@ -192,6 +192,11 @@ wxBitmap::wxBitmap(const wxSize& sz, int depth )
     Create(sz, depth);
 }
 
+wxBitmap::wxBitmap(int width, int height, const wxDC& dc)
+{
+    Create(width, height, dc);
+}
+
 // Create a wxBitmap from xpm data
 wxBitmap::wxBitmap(const char* const* bits)
 {