Measures text with the current TextStyle, including glyph overhang, in the same coordinates as Painter.Text. maxSize supplies the same layout bounds as a drawing rectangle; default uses Painter.Text's unconstrained bounds. The result includes ScaleToScreen but excludes Painter.Transform, clipping and opacity. Empty text returns zero. Prepares cached layout without recording a draw or allocating a bitmap or GPU texture. A later draw reuses the layout.