Compute the logical and ink extents of a glyph string.
See the documentation for [[email protected]_glyph_extents] for details about the interpretation of the rectangles.
Examples of logical (red) and ink (green) rects:
 
| this |
a `PangoGlyphString` |
| font |
a `PangoFont` |
| ink_rect |
rectangle used to store the extents of the glyph string as drawn |
| logical_rect |
rectangle used to store the logical extents of the glyph string |