Support for Mac OS X 10.5 Leopard

CGLShareGroupObj and IOSurfaceRef were added in 10.6 Snow Leopard SDK
Add workarounds to glimports.hpp to support building for pre-10.6.

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Regal currently supports 10.5 onwards, and I personally like to use the old gcc compiler toolchain that is faster than clang.  This modest patch is necessary for apitrace to build with the 10.5 SDK. (ppc/i686/x86_64)

- Nigel