[cff] Add early exit feature for width-only calls.

This is for `FT_Get_Advance'.

There are 7 places where the spec says the width can be defined:

  hstem/hstemhm
  vstem/vstemhm
  cntrmask/hintmask
  hmoveto
  vmoveto
  rmoveto
  endchar

* src/cff/cf2intrp.c (cf2_doStems): Exit early for width-only calls,
if possible.

(cf2_interpT2CharString) <cf2_cmdHSTEM>, <cf2_cmdVSTEM>,
<cf2_cmdVMOVETO>, <cf2_cmdENDCHAR>, <cf2_cmdHINTMASK>,
<cf2_cmdRMOVETO>, <cf2_cmdHMOVETO>: Exit early for width-only calls.