Widen data type for tta:{gain,pan} (#955).

[skynavga]

Closes #955.

[nigelmegitt]

I would prefer not to do this but approving for now since it is non-normative.

The impact of not doing it is that there would be some false positive validations, potentially, where values like 0. would be accepted when they should not be, as per the issue. I don't think that's particularly a bad thing, compared to what this pull request does, which is to allow a huge amount of invalid values unless validator code does further checking.

By the way, there's a general systemic issue with all the styling namespaces attribute values that are not [xs:]string, in that they always generate false negatives when used in the context of an <animation-value-list>, because the semicolon delimiter is not permitted in the schema, though it is in the spec.

[skynavga]

Merged early per WG resolution.

[skynavga]

@nigelmegitt re: your last paragraph of #956 (review), I have just checked the spec and schemas, and all continuously animatable styles except for tts:opacity are currently defined with a type of xs:string; therefore, to avoid false negatives during schema evaluation when using tts:opacity on tt:animate, we should change the type used with tts:opacity to xs:string in the schemas.