Fix creating CGContext from drawing context

Save initial affine transformations for just created CGContext
(like it's done in all wxMacCoreGraphicsContext ctors) to properly
manage user-defined transformations in SetTransform(),
GetTransform().

src/osx/carbon/graphics.cpp

@@ -1814,6 +1814,7 @@ bool wxMacCoreGraphicsContext::EnsureIsValid()
                 }
             }
 #endif
+            m_initTransform = CGContextGetCTM(m_cgContext);
             CGContextConcatCTM( m_cgContext, m_windowTransform );
             CGContextSetTextMatrix( m_cgContext, CGAffineTransformIdentity );
             m_contextSynthesized = true;