compileOTF produces Type2 CharString that exceeds the stack size limit

[irori]

(Filing here because I'm not sure which dependent library should be fixed.)

This program generates a font which reproduces the problem:
https://gist.github.com/irori/b0d5f53b6298efab4e526d68be2eab8d

Generated font is rejected by OTS because its Type2 Charstring exceeds the argument stack depth limit (48).

The above test program produces Type2 Charstrings like this:

[push 2 numbers] RMoveTo
[push 48 numbers] RLineTo
-90 -100 RMoveTo
10 10 10 10 10 10 10 10 10 VLineTo
EndChar

After RLineTo are common to all glyphs, so compreffor subroutinizes it, resulting in:

[push 2 numbers] RMoveTo
[push 48 numbers] [push 1 number] CallSubr

This exceeds the stack size limit.

Possible fixes:

• Make compreffor not insert CallSubr where the stack is full, or
• Reduce maxstack for fontTools.cffLib.specializer.specializeCommands by 1, so that there's always a room for a subroutine number.

[behdad]

Reduce maxstack for fontTools.cffLib.specializer.specializeCommands by 1, so that there's always a room for a subroutine number.

This makes sense to me.