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xlib:create-glyph-cursor -- a spec issue

I stumbled on a minor lacuna in the CLX spec for create-glyph-cursor.  What is
supposed to happen when mask-font is specified but mask-char is omitted?  The
code just wires mask-char=0, which doesn't seem particularly useful. Several
alternatives are possible.

1. Signal error
2. Default to source-char

3. Default to 0

4. Default to (font-default-char mask-font)

5. If (eq source-font mask-font), default to (1+ source-char), else signal

I suggest #5.  Since the source/mask-char's have to be tightly coupled, it's not
reasonable that any single value could serve as a default mask-char, so #2 is
plausible but #1 is more correct.  But #5 also allows a short-hand for a common
idiom based on a special font structure (like that of the "cursor" font).