Performance: add optional downscaling before applying filter

[reloxx13]

…lters for improved performance

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "x" next to an item)

• Documentation update
• Bug fix
• New feature
• Other, please explain:

What changes did you make? (Give an overview)

Add optional downscaling before applying filters for improved performance on low-end devices

Is there anything you'd like reviewers to focus on?

[reloxx13]

Needs testing on live system

[reloxx13]

on a pi3b+
from 3-5s depending on filter and ogrinal 1920px width
to 1-3s with max width 800px
great time improvement and the slow pi and photoquality is still fine on tablet and printed.

Limited to 800px
Average: ≈ 1,546 ms (≈ 1.55 seconds)
Filters: 25
Total processing time: 38,661 ms
Notable outlier: selective-blur (5,806 ms), which significantly increases the average.

Original 1920px
Average: ≈ 4,265 ms (≈ 4.27 s)
Filters: 26
Total processing time: 110,892 ms
Biggest outlier: selective-blur = 28,729 ms (also blur = 12,474 ms)
Average without selective-blur: ≈ 3,286.52 ms (≈ 3.29 s)

800px results

``` ["Processing photo took 1341ms for filter `antique`"] ["Processing photo took 1226ms for filter `aqua`"] ["Processing photo took 1179ms for filter `blue`"] ["Processing photo took 2979ms for filter `blur`"] ["Processing photo took 1244ms for filter `color`"] ["Processing photo took 1545ms for filter `cool`"] ["Processing photo took 1486ms for filter `edge`"] ["Processing photo took 1490ms for filter `emboss`"] ["Processing photo took 1304ms for filter `everglow`"] ["Processing photo took 1182ms for filter `grayscale`"] ["Processing photo took 1198ms for filter `green`"] ["Processing photo took 1506ms for filter `mean`"] ["Processing photo took 1208ms for filter `negate`"] ["Processing photo took 1192ms for filter `pink`"] ["Processing photo took 1166ms for filter `pixelate`"] ["Processing photo took 1199ms for filter `red`"] ["Processing photo took 1247ms for filter `retro`"] ["Processing photo took 5806ms for filter `selective-blur`"] ["Processing photo took 1232ms for filter `sepia-light`"] ["Processing photo took 1278ms for filter `sepia-dark`"] ["Processing photo took 1486ms for filter `smooth`"] ["Processing photo took 1341ms for filter `summer`"] ["Processing photo took 1268ms for filter `vintage`"] ["Processing photo took 1367ms for filter `washed`"] ["Processing photo took 1191ms for filter `yellow`"] ```

Original 1920px results

``` ["Processing photo took 2113ms for filter `plain`"] ["Processing photo took 2864ms for filter `antique`"] ["Processing photo took 2470ms for filter `aqua`"] ["Processing photo took 2377ms for filter `blue`"] ["Processing photo took 12474ms for filter `blur`"] ["Processing photo took 2564ms for filter `color`"] ["Processing photo took 4409ms for filter `cool`"] ["Processing photo took 3970ms for filter `edge`"] ["Processing photo took 3989ms for filter `emboss`"] ["Processing photo took 3031ms for filter `everglow`"] ["Processing photo took 2305ms for filter `grayscale`"] ["Processing photo took 2388ms for filter `green`"] ["Processing photo took 4014ms for filter `mean`"] ["Processing photo took 2306ms for filter `negate`"] ["Processing photo took 2386ms for filter `pink`"] ["Processing photo took 2206ms for filter `pixelate`"] ["Processing photo took 2383ms for filter `red`"] ["Processing photo took 2628ms for filter `retro`"] ["Processing photo took 28729ms for filter `selective-blur`"] ["Processing photo took 2580ms for filter `sepia-light`"] ["Processing photo took 2881ms for filter `sepia-dark`"] ["Processing photo took 3872ms for filter `smooth`"] ["Processing photo took 3255ms for filter `summer`"] ["Processing photo took 2821ms for filter `vintage`"] ["Processing photo took 3476ms for filter `washed`"] ["Processing photo took 2401ms for filter `yellow`"] ```

[reloxx13]

Could add a function to reuse the downscaled image if the user tries different filters, but this should be fine for now. Will need the other delete PR merged before, because the resized image has to be deleted too.

[andi34]

Rebase after latest config handling changes. Also squashed 2 commits for cleaner commit history.

[reloxx13]

rebased
are you fine with merging @andi34 ?