Fix all composer related tests (plus a bonus feature)

The composer test suit is in bad shape at the moment. This pull request fixes all composer related tests which are currently failing. First it reworks qgscompositionchecker to derive from renderchecker (#9117), so that anomaly images can be set for composer tests (the old method of allowing a set number of differing pixels was too inflexible and allowed tests to pass when they shouldn't be, eg the composermap zebra style test). Then I've reworked all the compositionchecker related tests so that they all pass, in some cases adding anomaly images or fixing the underlying bugs.

It also adds a new feature required to fix the failing zebra style test (#8767), which is the ability to set the pen and colours used in zebra style maps.  

The composer html tests had to be fixed by forcing the test html file to use a set pixel based font size and the expected images regenerated. This may mean they will currently fail on non-linux platforms, but fixing this will be a simple matter of adding anomaly images for these platforms.