TransparencyTool: don't check sqrt table when using current brush

[dsizzle]

• a user brush can have values outside the bounds of the sqrt table, so check if we're using a brush before checking the sqrt table.

Fixes #529

...I knew about this issue and then forgot. Glad you caught it before the next release!

[humdingerb]

Uhhhh, thinking about a next release... Exciting! :)

[dsizzle]

Well, not sure how soon. Every time we do a release I start thinking about the next one!!! 😉

[humdingerb]

That's the spirit! :)