CI update, general fixes and clap

[falkTX]

I meant to just update the github CI file, but ended adding a couple more changes.

In this PR:

• CI is fixed to work with latest github VMs
• linux-riscv64 builds added to CI and releases
• removed cairo and dbus from CI, not needed
• updated plugin-validation kxstudio repos, for new lv2 related things
• added "self-test" during plugin validation step, which checks for DPF API misusage and standalone memory leaks
  (sadly this fails on CI right now, dont know why yet as it works for other plugins and locally too, will investigate more later)
• fix a small memory leak detected by the self-test
• tag all audio ports as stereo, which helps VST3 hosts
• set default size macro based on artwork background size, helps in a few VST2/VST3/CLAP hosts (we can skip creating a temporary dragonfly UI just to know its size)
• initial clap support, still experimental but already works well

The VST3 support should be finalized in DPF side, which this PR updates to.

Latest DPF has initial CLAP support, enabled here too

[falkTX]

Correction: the clap related issues are now sorted

[michaelwillis]

Thanks @falkTX!

[falkTX]

thanks for merging. might be nice to merge HiDPI branch to master too

[michaelwillis]

Right, I never figured out the AU build, but I should do that on a different branch.