Improve the composition performance on GTK

[yvt]

• [ ] swrast needs optimization.
• [ ] See if GSK (GTK Scene Graph Kit; added in GTK 4) is up to the task
• [x] ~~Cairo~~ It doesn't support 9-slice scaling
• [ ] Add a Vulkan compositor if GSK can't cover our use case - VK_KHR_incremental_present