Gets the extents for an array of glyphs. The extents describe a user-space rectangle that encloses the "inked" portion of the glyphs, (as they would be drawn by showGlyphs()). Additionally, the x_advance and y_advance values indicate the amount by which the current point would be advanced by showGlyphs().

Note that whitespace glyphs do not contribute to the size of the rectangle (extents.width and extents.height).