Heal Transparency fails to heal semi-transparent pixels: error message "texture source is empty"

[bootchk]

Reproduce:

1. Add alpha channel
2. User Eraser tool with a soft brush (say hardness 50%)
3. Filters>Heal Transparency, click OK

Expect: the erasure is healed.

Actual: error message "texture source is empty"

Discussion

The wiki says 'heal all transparent pixels'. But there are degrees of transparency, and the Heal Transparency plugin
only heals fully transparent pixels.

A fix would be for the plugin to "threshold alpha" before converting the alpha channel into a selection.
Then it would heal all pixels, even with partial transparency (except it might miss a very few, since the threshold would be 0.999)

The plugin is removing the alpha channel anyway.

This won't change the behavior on most other use cases, such as rotating an image,
since that leaves only fully transparent pixels, that are healed already.

After this change, the wiki would say "heals all pixels with any degree of transparency."
And suggest the use case using the Eraser Tool.
Some users might prefer to use the Eraser Tool instead of a Selection Tool
to designate what is to be healed.