add convertToWorkingRGBProfile option to getPixmap

By default, generator dumps out "raw" RGB values to pixmaps. If the document uses a color profile other than sRGB, this will result in output that looks different than the PSD (since the output doesn't have a color profile and the user's system will probably use sRGB to interpret it.)

Most of the time, web designers don't color manage their documents, so Generator usually does what they want. But, there's an option in `sendLayerThumbnail` that enables color conversion in these non-standard use cases. This pull request bubbles that option up to getPixmap.

A PR in generator-assets is forthcoming that adds a config option to leverage this. Automated tests are also forthcoming.

I thought this was what @dirkschulze was asking for in #249 but it turns out it isn't. So, I don't think anyone actually wants this. So, I spent my weekend implementing it -- I endeavor to make potential future users happy instead of current users.

@iwehrman maybe you can spend your weekend reviewing it!