This is submitted to 15.2 as well. This SR to 15.3 directly is to have it quicker in 15.3, to fix openQA failures.

- Add patch to fix software rendering detection with Mesa 20.2+
(boo#1179845):
* 0001-Detect-softpipe-and-llvmpipe-on-Mesa-20.2.patch

- Update to 5.18.6
* New bugfix release
* For more details please see:
* https://kde.org/announcements/plasma-5.18.6
- Changes since 5.18.5:
* x11: Make removal of X11 event filters safe (kde#423319)
* Fix invalidation of cached x stacking order in wayland only mode
* effects/slidingpopups: Properly clip windows
* Grab all possible keyboard modifiers for window commands (kde#424272)
* [x11] Hold a passive grab on buttons only when needed (kde#394772)
* Check if we successfully restored input focus
* Partially revert a0c4a8e766a2160 (kde#424223)
* [x11] Force FocusIn events for already focused windows
* [x11] Create egl scene textures with right size
* Fix the Plastik decoration with Qt 5.15
* [wayland] Fix misuse of EGL/eglmesaext.h (kde#422131)
* [wayland] Place lockscreen greeter above other windows
* [kcmkwin/kwindecoration] Don't exec() QDialog (kde#421053)