add brightness and contrast (#396)

* Imlib2 supports modifying gamma, brightness and contrast directly
  while sxiv only supports gamma. Makes sense to extend it to brightness
  and contrast as well.

* Since color corrections need to be aware of each other, they have been
  refactored into one centralized function.

* This also makes the code more hackable as it makes it easier to add
  more color correction functions without them interfering with each
  other.

Co-authored-by: 0ion9 <finticemo@gmail.com>
Co-authored-by: NRK <nrk@disroot.org>
Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/396
Reviewed-by: NRK <nrk@disroot.org>
Reviewed-by: TAAPArthur <taaparthur@noreply.codeberg.org>
Co-authored-by: Berke Kocaoğlu <kberke@metu.edu.tr>
Co-committed-by: Berke Kocaoğlu <kberke@metu.edu.tr>

etc/nsxiv.1

@@ -209,6 +209,26 @@ steps.
 .TP
 .B Ctrl-g
 Reset gamma correction.
+.TP
+.B [
+Decrease brightness correction by
+.I count
+steps.
+.TP
+.B ]
+Increase brightness correction by
+.I count
+steps.
+.TP
+.B (
+Decrease contrast by
+.I count
+steps.
+.TP
+.B )
+Increase contrast by
+.I count
+steps.
 .SS Thumbnail mode
 The following keyboard commands are only available in thumbnail mode:
 .TP