GafferImage Quick Fixes

These are just 2 quick things I needed to fix for playing with `GafferImage` nodes fed by `ExecutableRenders`.

- `ChannelMaskPlugValueWidget` now scopes the context before getting the channel names.
- `ImageWriter` now creates the necessary directories before writing the file

I guess it could be argued that the last one should be controlled by a bool plug, but `ExecutableRenders` happily make directories without exposing the option, so it seemed fair that `ImageWriter` could do the same.

Note that `ImageWriterTest.testPassThrough()` is currently failing, but it's failing on master as well (at least at IE). Oddly enough, reproducing the test in a gui session seems to work fine, but reproducing it from `cli` also fails...