text: Text fields without fonts should be 12pt sans

This is the default value emitted by the Flash authoring environment,
and also what the Flash Player does.
text: Don't skip rendering for initially-empty DefineEditText fields

For dynamic text fields, hasText only provides the initial value.
User code may set the field to say something else eventually.
TextField: Add support for replaceText
Merge branch 'text' of git://github.com/magcius/shumway
Merge branch 'master' of github.com:mozilla/shumway
Removes toDataURL operation from image mask code