Allow resizing images to keep full image instead of cropping

[makermelissa]

Add options to process_image so images can be added on top of background rather than just cropped. I think now that we have bitmap tools, this may be possible, though the images downloaded via the image formatter service are being loaded as ondiskbitmaps to conserve memory and bitmap tools requires the bitmaps to fully be loaded into memory.

Another option (and perhaps it already exists since there doesn't appear to be documentation) is to add a parameter to the image formatter service to let it know what color of background we would like and that would solve the issue.

See https://forums.adafruit.com/viewtopic.php?p=1014450 for more info.