When a subroutine contains no explicit hint stem operators (has_hint=False),
but it contains a hintmask operator which, in the context of the calling
charstring, would be understood as implying a vstemhm, then we need to set
has_hint=True on the subroutine, and update the last_hint index.
Otherwise, the drop_hints function leaves behind the arguments of the implicit
vstemhm operator.
This case is exemplified in the test font Tests/subset/data/Lobster.subset.ttx,
for charstrings "B" and "B.salt", and subroutine index="2".
--- /Users/cosimolupo/Documents/Github/fonttools/Tests/subset/data/expect_no_hinting_CFF.ttx
+++ /var/folders/jb/rjz76yw92w7144mwqg119jnm0000gn/T/tmpO_XOWh/tmp3.ttx
@@ -47,7 +47,7 @@
107 return
</CharString>
<CharString index="2">
- 230 636 rmoveto
+ 119 230 636 rmoveto
-136 -636 rlineto
144 hlineto
return
@@ -94,7 +94,7 @@
endchar
</CharString>
<CharString name="B">
- 187 -105 callsubr
+ 187 6 93 362 139 -119 101 -101 -105 callsubr
82 383 rlineto
2 18 20 1 8 hhcurveto
73 22 -57 -70 hvcurveto
@@ -109,7 +109,7 @@
endchar
</CharString>
<CharString name="B.salt">
- 185 -105 callsubr
+ 185 6 93 350 149 -119 105 -105 -105 callsubr
6 30 rlineto
-41 39 41 -17 39 hhcurveto
125 110 175 136 72 -32 62 -82 15 hvcurveto
2b76efb4ad