GLdispatch: Simplify the dispatch stub bookkeeping.

Changed the interface in stub.h to work with function indexes instead of
mapi_stub pointers. The mapi_stub structure is now an internal detail in
stub.c.

The mapi_stub struct now only contains a name and a slot.

Removed the dynamic_stubs array. Instead, it just keeps an array of the
function names for each dynamic stub. It now looks up the function pointers
with entry_get_public, and the slot number is always equal to the index, so the
name is the only thing it needs to keep track of.