The parser got its chars mixed up while parsing numbers
like 2.3e-04. While it is unlikely to meet such numbers
in human-generated css, we do have them e.g. when saving
render node trees with transforms.
Also add some css parser tests for number parsing.
CSS does not do exponents, so printing numbers close to 0 as 1.234e-15
does not work.
Also up the accuracy to 17 digits because that's what everyone else
uses.
Instead of an error vfunc, have the tokenizer vfunc take a GError
argument. Note that even when an error is returned, there is still a
token to be read.
