CPU demosaicing crashes on Windows 8

The problem seems to be with the GCC OMP (auto parallelising) support. If I disable that, it doesn't crash. As a short term fix, OMP should be disabled in the Windows build.

Since OMP doesn't work on Mac either, it should be replaced by explicit multi-threading of the CPU demosaicing impemented in python with multiple thread objects processing subsets of the image. That would then be portable to all platforms.