osd: Export some classes and methods for Windows DLL

This enables CPU, OpenMP, GLSL, and GLSL TransformFeedback kernels for dynamic
linking on Windows.
osd: Mark std::vector templates for exports

This fixes C4251 warnings.
C4251: 'identifier' : class 'type' needs to have dll-interface to be used by clients of class 'type2'

Note: The template instantiation has to be done outside the OpenSubdiv
namespace, otherwise it'll break the build on Linux gcc.

Note: This warning may actually be safe to ignore in the case of std::vector,
because std::vector contains only inline methods, and has no static members.
TEMP: favor dynamic GLEW

TODO: Need a way to decide which one to use.