Fixes https://github.com/fonttools/fonttools/issues/457
Backslash-prefixed glyph name can be used in a Feature file to distinguish them from identically-named keywords.
From section "2.f.i. Glyph name" of Adobe's Feature File Specification:
> An initial backslash serves to differentiate a glyph name from an identical keyword in the feature file language. For example, a glyph named "table" must be specified in the feature file as: \table
Thus, when we parse a glyph name that begins with a backslash, we need to ignore the first character.
Note that makeotf rejects feature files with glyph names that start with or contain backslashes, even when escaped with another backslash.
feature liga {
sub \\glyphWithBackslash by A;
} liga;
This yields:
makeotfexe [FATAL] <Backslash-Regular> invalid token (text was "\") [features 2]
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