[feaLib.parser] strip initial backslash from glyph names
[feaLib.parser] strip initial backslash from glyph names

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]

googlefonts/fonttools,fonttools/fonttools