Improvements to osd_regression

Several things:

* Make it build on OSX (GLEW is not required there)
* Make it actually count errors
* Changes to support running more than just single threaded CPU backend. Currently implemented "CPU" (as it was before) and "CPUGL" (CPU kernels, backed by GL vertex buffer).
* Command line arg ("-backend all" or one of names, e.g. "-backend CPUGL") to run all or a specific backend. When not present, runs old behavior (CPU backend).

I wanted to also add OpenCL etc. backends, but currently they lack ability to read data back to the CPU; there's no equivalent of BindCpuBuffer in some of them. I could try adding that just for purposes of regression testing, but that would be a more invasive change. Thoughts?