Performance in Recolor Mode

[Matthias Möller]

Hi Okular Team,

I very much like your recolor mode ("Configure" -> "Accessibility" -> 
check "Change Colors" -> select "Change Dark & Light Colors" in combobox).

Unfortunately, scrolling a pdf in this mode is on my little notebook not 
that smooth, it is currently a bit sluggish.

A quick little benchmark session yesterday[1] showed me, that there are 
some easy improvements left in the `PagePainter::recolor` function. 
Improvements, which are used for similar functions like 
`PagePainter::invertLightness`.

Those changes lowered the runtime for the recolor function on my machine 
from around 30ms to 3ms. [1]

I am very confident, that this will solve the small performance problem 
during scrolling on my notebook, as the recolor can now be done in >24fps.


Attached to this E-Mail is also a patchfile with the small improvements.


Greets,

     Matthias M.


[1] Benchmark Gist: 
https://gist.github.com/TinyTinni/235527ffba9489ed29f7ba558e13724f


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