Fix wxGCDC::Clear (wxOSX)

If underlying graphics context is rotated then drawing a rectangle with origin at (0,0) doesn't cover all the drawing area. To draw over entire area we need to get extents of the actual clipping region (with applied all transformations) and use it as coordinates of the drawn rectangle.
Solution for wxMSW and wxGTK was implemented in 12eaa61212eaa61930.

See #17636.

src/osx/carbon/graphics.cpp

@@ -1906,8 +1906,6 @@ void wxMacCoreGraphicsContext::ResetClip()
 
 void wxMacCoreGraphicsContext::GetClipBox(wxDouble* x, wxDouble* y, wxDouble* w, wxDouble* h)
 {
-    // This function is not yet tested.
-    // TODO: Do the tests.
     CGRect r;
 
     if ( m_cgContext )