Fix wxBitmap::GetRawData() in wxQt

This method used to return a dangling pointer to a temporary buffer,
which resulted in a crash when using it, e.g. in the unit test.

Fix this by keeping a QImage as a member in wxBitmapRefData, so that the
pointer to its data remain valid until UngetRawData() is called.

Also check that GetRawData() returns a non-null pointer in the test.

Closes https://github.com/wxWidgets/wxWidgets/pull/1067

tests/graphics/bitmap.cpp

@@ -126,6 +126,7 @@ void BitmapTestCase::OverlappingBlit()
     if ( m_bmp.GetDepth() == 32 )
     {
         wxAlphaPixelData npd( m_bmp );
+        CPPUNIT_ASSERT_MESSAGE( "Expected raw pixels to not be NULL", npd );
         wxAlphaPixelData::Iterator it( npd );
 
         ASSERT_EQUAL_RGB( it, 255, 0, 0 );