Support `animation @fill`

[nigelmegitt]

The TTML2 animate element supports a fill attribute.

The default of "remove" means that at the end of the animation, the end value is reverted. In general we want the semantic associated with the alternative value, "freeze" to apply, where whatever the end value is gets retained.

For example:

        <div begin="0s" end="2.3s">
            <animate begin="0.0s" end="0.2s" tta:gain="1;0.4"/>
            <animate begin="2.1s" end="2.3s" tta:gain="0.4;1"/>

This varies the gain from 1 -> 0.4 between 0.0 and 0.2s, and then varies it from 0.4 -> 1 between 2.1s and 2.3s. But what happens between 0.2s and 2.1s? By default the gain value reverts to its default of 1, but we want to "freeze" it at 0.4 in this example:

        <div begin="0s" end="2.3s">
            <animate begin="0.0s" end="0.2s" fill="freeze" tta:gain="1;0.4"/>
            <animate begin="2.1s" end="2.3s" tta:gain="0.4;1"/>

Our current default behaviour is actually "freeze" but that's wrong - it should be "remove" according to the spec.