glBufferSubData() with a max size limit, to work around NVidia's threaded driver limits (anything > than roughly 256KB triggers a sync with the server

forbid default constructor.

Construct with a NULL function pointer. You must manually call Lookup() before you can call a dynamic function through this interface.

check: verify that context equals cache, return true if mismatched or if illegal values seen

default: write default value to cache, optionally write through