Scale filter and Downscale and Upscale as derivatives, with a new feature

[deviprsd]

Like I suggested in #770 Creating a new filter scale and deriving upscale and downscale from it will be much clear. Other than that, there are new options like by or to depending upon filter where a relative ratio or absolute ratio respectively can be set to up or down scale the image. For example,

filters:
    downscale: { max: [50, 70] } #or { by: 0.6 } -> Downscales to 40%
    upscale: { min: [200,null] } #or { by: 0.7 } -> Upscales to 170%
    scale: { dim: [600,750] } #or { to: 1.56 } -> Upscales to 156% | { to: 0.66 } -> Downscales to 66%

Though relative_resize has a scale method, the scale filter is more flexible than the latter.

[lsmith77]

can you update the docs as well?

[alexwilson]

Looks way nicer!

[deviprsd]

After giving tests a good look, I don't think new tests are required. If upscale and downscale work, the scale should be assumed to work.

[lsmith77]

thx