Created attachment 98157 [details]
rendering

Please look into attached screenshot. What you see there is reproducible with
enabled glamor and with ogl1/2/3/gles/xrender KWin compositing engine and
without compositing. For some reason this glamor issue is KWin specific, there
is no such artefacts with Compiz or Metacity.
Tested on Linux 3.11-3.14, Mesa 9.2-10.2 git (git1404271930.151a20), libglamor
0.5-0.6 git (git1403131830.a4fbc7), radeon driver 7.20-7.3.0 git
(git1404160730.48d3db), X.Org Server 1.14-1.15.
Kubuntu 13.10-14.04 x86_64, KWin 4.11.0-4.11.8, Radeon HD 6620G.