[Filter] Add "fixed-size" filter implementation

[peter-gribanov]

Q	A
Branch?	1.0
Bug fix?	yes
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#560
License	MIT
Doc PR	-

At the moment there is no way to create a fixed-size image.
This is a new filter that does this.

See #560 (comment) for more details.

Usage

liip_imagine:
    filter_sets:
        my_thumb:
            filters:
                fixed: { width: 120, height: 90 }

[lsmith77]

@antoligy / @robfrawley how do we want to deal with this .. 1.0 or 2.0 ?

[robfrawley]

My opinion is to have this as a new feature for our new 2.x major release. (Tagging as such now, but we can change back to 1.0 if others disagree)

[robfrawley]

@peter-gribanov Please update/add to the RST documentation to include information about the new filter implementation you are introducing here. Reference the recent PR #941, which adds a resolution filter, for information on the expected additions and updates that need to be included in this PR.

Note: While working on the RST docs, please be sure to test your changes and view the rendered output for correctness in a web browser of your choice. See the wiki entry for building the RST documentation for an overview of how to compile the RST files into HTML.

Otherwise, everything else looks great! Thanks for the contribution. Shout out to @robbeman for the general filter implementation here.

[peter-gribanov]

@robfrawley I added documentation about the filter. Maybe you want to add something.

[robbeman]

@robfrawley Thanks for the mention ❤️. (Yes, I'm that slow in noticing these things, hehe.)

[crabnky]

Any idea when this can be merged?

[peter-gribanov]

@maximgubar thanks 👍