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.

interface/wx/bitmap.h

@@ -306,6 +306,20 @@ public:
     */
     wxBitmap(const wxSize& sz, int depth = wxBITMAP_SCREEN_DEPTH);
 
+    /**
+        Create a bitmap compatible with the given DC, inheriting its magnification factor
+
+        @param width
+            The width of the bitmap in pixels, must be strictly positive.
+        @param height
+            The height of the bitmap in pixels, must be strictly positive.
+        @param dc
+            DC from which the scaling factor is inherited
+
+        @since 3.1.7 (previously available only in wxMSW and wxOSX ports).
+     */
+    wxBitmap(int width, int height, const wxDC& dc);
+
     /**
         Creates a bitmap from XPM data.