This is in preparation for calc(), as calc(50% - 5px) is valid CSS
but has 2 units. Instead, add a function to query a value's dimension
(so we can differentiate lengths from numbers) and add a function to
query if the value contains percentages.
Don't allow syntax like
at top left circle
but follow the spec about requiring the at <position> right before the
comma.
This is porbably because
circle at 10px 10px
could be interpreted as
circle 10px at 10px
with the now disallowed syntax, too.
Test included.
