#16 Variable references not resolved

[Richard Light]

If a variable reference refers to a variable which is
declared in a preceding sibling of an ancestor element
(i.e. not a preceding sibling of the element making the
reference) is cannot be resolved.

TXpXslContext.GetVar should iterate up the stylesheet
tree (possibly only if FUsePrevVars is true?) looking
for a "local" variable before it falls back to looking
for a global one.