Clarify default value of blur radius

[palemieux]

The default value of blur radius is not specified. It should be clarified to be zero.

[css-meeting-bot]

The Working Group just discussed TTML1, and agreed to the following resolutions:

• RESOLUTION: WG agrees the default blur radius is zero, and it should be clarified.

The full IRC log of that discussion

<nigel> Nigel: One issue was raised recently, to clarify the default value of blur radius
<nigel> .. Just to check, are we in agreement that the default value of blur radius should be zero?
<nigel> Glenn: That's what it is in practice, it was just an omission that we did not specify that.
<nigel> github: https://github.com//issues/351
<nigel> RESOLUTION: WG agrees the default blur radius is zero, and it should be clarified.