Bug 18787: Remove unnecessary code from glyphFont()

[hturner]

As discussed in Bug 18787 R Core welcomes a patch for grDevices::glyphFont():

• Modify glyphFont() so that it only produces a single font description.
• Add some simple tests of the above to grDevices/tests/
• May need clarifications within ?glyphFont
• Regression test grid/tests/glyphs.R (including checks that output is unchanged, e.g., via {gdiff}).
• Check that the changes do not break {marquee} (or change its output).

The file paths above refer to relative paths in the R sources, which you can browse on the r-svn GitHub mirror.