The tiny patcher can be used to remove black bars from the game at 21:9 and wider resolutions.
Follow the instructions within the tool to patch the specified game file.
If necessary, any resolution can be set in
%USERPROFILE%\AppData\LocalLow\Weappy\Rebel Cops\Settings.xml
(open in Notepad)
Tested the latest GOG version briefly at 2560x1080, 3840x1080 and 5760x1080. The menu cursor disappeared at the triple monitor resolution.