Launching marco-compton.desktop ends with /usr/bin/marco-compton: line 31: compton: command not found in ~/.xsession-errors

[Norbert]

Steps to reproduce:
1. Boot Ubuntu MATE 20.10 daily ISO
2. Launch marco-compton.desktop from /usr/share/applications while controlling the log in ~/.xsession-errors

Expected results:
* application launched, log is empty

Actual results:
* application is not launched log contains line:

```
/usr/bin/marco-compton: line 31: compton: command not found

```

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: mate-tweak 20.04.0-2
ProcVersionSignature: Ubuntu 5.4.0-42.46-generic 5.4.44
Uname: Linux 5.4.0-42-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu43
Architecture: amd64
CasperMD5CheckMismatches: ./casper/filesystem.squashfs
CasperMD5CheckResult: skip
CasperVersion: 1.452
CurrentDesktop: MATE
Date: Thu Aug 6 10:15:52 2020
LiveMediaBuild: Ubuntu-MATE 20.10 "Groovy Gorilla" - Alpha amd64 (20200806)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: mate-tweak
UpgradeStatus: No upgrade log present (probably fresh install)

[Norbert (nrbrtx)]

The compton package is not installed, as it is only suggested by mate-tweak:

```
$ apt-cache policy compton
compton:
  Installed: (none)
  Candidate: 0.1~beta2+20150922-2
  Version table:
     0.1~beta2+20150922-2 500
        500 http://archive.ubuntu.com/ubuntu groovy/universe amd64 Packages

$ aptitude why compton
i ubuntu-mate-desktop Recommends mate-tweak
i A mate-tweak Suggests compton
```

[Ubuntu QA Website (ubuntuqa)]

This bug has been reported on the Ubuntu ISO testing tracker.

A list of all reports related to this bug can be found here:
http://iso.qa.ubuntu.com/qatracker/reports/bugs/1890570