Failed to generate dynamic library of libgpu

[peyer]

@rexbu I have successfully compiled files under libGPU/bs to generate library of libbs.
However, when I tried to compile files under libGPU/filter, it shown that some files rely on libGPU/base, so I tried to compile files under libGPU/base, it just failed by undefined reference to 'vtable for GPUContext' in GPUContext.cpp file. I noticed that virtual member func createContext/ ~GPUContext() / swapBuffer(void* surface=NULL) of class GPUContext do not implemented in corresponding cpp file, does that right ? Hope for your reply