#36 gimp_channel_new has no precision argument

[Sam Richards]

The gimp_channel_new that is used inside plugins (such
as file-loaders) for creating new channels, does not
have an argument for specifying what the default
precision is.

This means it currently defaults to FLOAT16, which is
not quite right for some of the work we do.

We will need to fix this if we want to use channels to
store the alpha.