Add wxBitmap::SetScaleFactor()

We need to be able to change the scale factor of the bitmaps returned by
wxBitmapBundle::GetBitmap(), so add a function allowing to do this.

Also add wxHAS_BITMAP_SCALE_FACTOR allowing to check whether this
function actually does something non-trivial and explain in the docs
that GetScaleFactor() always returns 1 on the platforms where this
symbol is not defined.

docs/doxygen/mainpages/const_cpp.h

@@ -167,6 +167,9 @@ Currently the following symbols exist:
     have an efficient (CPU-specific) implementation. Notice that the functions
     themselves are always available but can be prohibitively slow to use when
     implemented in a generic way, using a critical section.}
+@itemdef{wxHAS_BITMAP_SCALE_FACTOR, Defined in @c wx/bitmap.h if bitmaps
+    actually use scale factor under the current platform, see
+    wxBitmap::SetScaleFactor().}
 @itemdef{wxHAS_BITMAPTOGGLEBUTTON, Defined in @c wx/tglbtn.h if
     wxBitmapToggleButton class is available in addition to wxToggleButton.}
 @itemdef{wxHAS_CONFIG_TEMPLATE_RW, Defined if the currently used compiler