Blender Git Loki

Compositor: implement OpenCL backend for gaussian blur

Pretty much straightforward change which gives around 30%
speedup on my laptop and around 2x speedup on desktop in
the BI (which uses gts580). Tested with huge blurs (like
10% of blur) which was rather common during Caminandes.

For now OpenCL is only limited for blur size more than
100 pixels.

This is a bit experimental still, feedback is welcome.

Reviewers: jbakker, lukastoenne

Subscribers: ton

Differential Revision: https://developer.blender.org/D576