Check if stack of graphics states is empty while attempting to reset the clip

For macOS < 10.13 there is not available any method to really reset
the clipping region so ResetClip() is implemented using hack that
works for sure only if no graphics state was stored before on the stack
with PushState(). If ResetClip() is called when some graphics states
are already stored with PushState() we can only warn that resetting the clip
may be not effective.

See #19231.

src/osx/carbon/graphics.cpp

@@ -2141,6 +2141,12 @@ void wxMacCoreGraphicsContext::ResetClip()
         {
             // there is no way for clearing the clip, we can only revert to the stored
             // state, but then we have to make sure everything else is NOT restored
+            // Note: This trick works as expected only if a state with no clipping
+            // path is stored on the top of the stack. It's guaranteed to work only
+            // when no PushState() was called before because in this case a reference
+            // state (initial state without clipping region) is on the top of the stack.
+            wxASSERT_MSG(m_stateStackLevel == 0,
+                         "Resetting the clip may not work when PushState() was called before");
             CGAffineTransform transform = CGContextGetCTM( m_cgContext );
             CGContextRestoreGState( m_cgContext );
             CGContextSaveGState( m_cgContext );