Gets the horizontal component of the glyph's advance in device space: the text-space advance (AdvanceX,
AdvanceY) mapped by the text matrix T_m and the CTM, so the next glyph's DeviceOrigin is this
one's plus the advance (when no TJ number, spacing change or text positioning comes between them).
public double DeviceAdvanceX { get; }
ISO 32000-2 §9.4.4: the advance translates T_m; §8.3.2.3: the CTM maps user space to device space.