picom needs a sane default configuration

With bug 1919370 we will be changing from compton to picom. It doesn't come with a default configuration and though picom can load a compton.conf, it doesn't support all of its options, so we need to adapt it accordingly.

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: lubuntu-default-settings 21.04.3
ProcVersionSignature: Ubuntu 5.10.0-14.15-generic 5.10.11
Uname: Linux 5.10.0-14-generic x86_64
ApportVersion: 2.20.11-0ubuntu60
Architecture: amd64
CasperMD5CheckMismatches: ./casper/filesystem.manifest
CasperMD5CheckResult: fail
CasperVersion: 1.460
CurrentDesktop: LXQt
Date: Tue Mar 16 19:23:23 2021
LiveMediaBuild: Lubuntu 21.04 "Hirsute Hippo" - Alpha amd64 (20210311)
PackageArchitecture: all
SourcePackage: lubuntu-default-settings
UpgradeStatus: No upgrade log present (probably fresh install)