Separate EWMH colour conversion from icon generation.

Separate EWMH colour conversion from icon generation.

We don't need to recompute the ARGB colour values for every single
pixel, we can do this in advance once for each possible colour.  To
do this, we alter the behaviour to use the 'pixel' member of the
supplied XColor array, and create a new function that can be called
beforehand to populate it based on red/green/blue values.

Interestingly, the ewmhicon test program was already providing an
XColor array with 'pixel' values that are correct for this new
behaviour, so it needs no changes, except a new mode is added
as a basic sanity check for the new function.