Improve trimming of images with transparency

[muse-monotype]

Retrieve the background colour alpha value and pass image alpha band to find_trim() if background colour is not fully opaque. Attempt to fix issue #1597

[coveralls]

Coverage remained the same at 99.756% when pulling 8455963 on Monotype:master into fb5c393 on lovell:master.

[lovell]

Thanks for the PR. Am I correct in thinking that if the top left pixel has any transparency then only the alpha channel is used for the trim? I'm not sure this is the desired behaviour, or at least could break backwards compatibility.

Perhaps a two-step process could be used instead, where if the initial call to find_trim returns zero offsets and the image has an alpha channel, then perform a second call on just the alpha channel.

[Insality]

Any info about this PR? Have the same trouble with black and transparency and this PR will solve it

And I can't make working package from @muse-monotype fork working on windows (have no software to build-from-source it)

[lovell]

I've addressed this in commit c41b873, which adds support for trimming images where the alpha channel contains the "data" (and non-alpha channels are a single colour) and maintains backwards compatibility with existing behaviour.

@muse-monotype Thank you very much for taking the time to submit the original PR Martin, these are always highly appreciated.