Roadmap to Vulkan

Vulkan brings some advantages like async compute, better defined behavior and in general more control over the specifics of what's happening. This is a rough plan on how to get there, and how to do it in steps, without having one giant merge request

Basic infrastructure

• require Vulkan 1.3 and all the extensions that make our lives easier. We have OpenGL as a fallback, so compatibility can effectively be ignored
• add Vulkan support to the Wayland backend, to create a device and allow for testing
• export texture to OpenGL and use OpenGL compositing with a Vulkan backend at first
• port shaders to Vulkan, add CMake command to compile it to SPIR-V at built time
• scene: paint items, without any effects as a start

Libkwineffects

• add infrastructure for Vulkan objects
• add infrastructure for Vulkan <-> OpenGL interop
• port offscreen effects to use Vulkan <-> OpenGL interop
• import Vulkan textures into QtQuick
• port:
  • contrast to Vulkan
  • blur to Vulkan
  • colorpicker to Vulkan
  • invert to Vulkan
  • magnifier to Vulkan
  • mouseclick to QML
  • mousemark to QML
  • screenedge to QML
  • screenshot to Vulkan
  • screentransform to Vulkan
  • showpaint to Vulkan
  • snaphelper to QML
  • touchpoints to QML
  • trackmouse to QML
  • zoom to Vulkan
  • startupfeedback to Vulkan or QML

Other plugins

• qpa: port to Vulkan, or use OpenGL interop
• screencast: port to get a buffer instead of a texture from the backend -> screencast code can mostly stay untouched besides that

Other backends

• virtual backend: should be very straight forward, it's just one texture
• drm backend: write a replacement for EglGbmBackend + EglGbmLayer. To simplify multi gpu, only support setups where all GPUs support Vulkan

Long term goals

• make use of asynchronous compute for simple scenes (no active effects)
• port effects from Vulkan <-> OpenGL interop to use Vulkan directly?
• port shader loading to Qt's shader infrastructure? (QTBUG-113331)