Create skylines directly from glyph outlines.

Create skylines directly from glyph outlines.
    
    This avoids creating garbage for the intermediate Scheme data to
    transport the glyph outlines.
    
    a20f8b786e - Create skylines directly from glyph outlines.
      baseline: 267e894e79 Thread skyline construction through stencil interpretation
      args: input/regression/mozart-hrn-3
      memory: med diff 76 (stddevs 77 85, n=3)
      memory: med diff 0.0 % (a20f8b786e is neutral)
      time: med diff 0.02 (stddevs 0.02 0.02, n=3)
      time: med diff 0.6 % (a20f8b786e is neutral)
    
    a20f8b786e - Create skylines directly from glyph outlines.
      baseline: 267e894e79 Thread skyline construction through stencil interpretation
      args: -I carver MSDM
      memory: med diff -19408 (stddevs 134 228, n=5)
      memory: med diff -1.8 % (a20f8b786e is leaner)
      time: med diff -1.14 (stddevs 0.10 0.14, n=5)
      time: med diff -2.3 % (a20f8b786e is faster)

[hanwenn, 2020-04-26 07:44:58]

goes on top of https://codereview.appspot.com/581960043/

[hanwenn, 2020-04-26 08:30:36]

rebase

[hanwenn, 2020-05-02 07:34:10]

rebase

[hanwenn, 2020-05-02 21:49:11]

rebase