api: fix best fit for thumbnail, add background

[kpsherva]

• fixes for best fit case (f.e. dimensions !400,200) - changed variable in the formula

• expected behaviour for best fit case changed

  • now it resizes the image and keeps the ratio of an image and fills the background to requested size with black pixels - this helps to keep responsive images in the UI

• adds the function to fill the pixels with black background if the best fit is smaller than requested thumbnail box size - preserves the thumbnail ratio

• add the function to fill the background with black pixels for GIF if the best fit is smaller than requested window size

• provides testing cases for portrait, landscape and fixed best fit case

• related to requested fix: (closes #47).

[drjova]

@kprzerwa thank you for your contribution, could you please rebase?

[drjova]

Could you please also add a release commit for version v0.5.0, example

[kpsherva]

@drjova thank you for the review, is it all ready to merge?