[Beignet] Beignet vs LuxRays, again

[Simon Richter]

Hi,

I've just rendered the first picture using LuxRays and Beignet.
Observations:

 - During rendering, colored dots appeared all over the display at
random locations
 - In the final result, there are black pixels
 - Fonts in some applications appear garbled
 - Rendering was slow (AMD's CPU-based OpenCL implementation was
significantly faster on the same machine, an i7-3612QM)

I think there may be something wrong with the buffer management here, if
the on-screen buffer and off-screen textures owned by different apps can
be overwritten by OpenCL code.

   Simon