New Feature: Ability to "get_proc_address" with *const c_char directly.

[Lokathor]

All the actual C level functions to get_proc_address accept *const c_char, not &str.

In the current implementation, a user has to pass &str, then the library converts that to a CString, and then uses the pointer to that to perform the actual OS call.

Since the GL function names are stored as const values in the loader, the loader can just add the '\0' and then pass a *const c_char directly, and avoid doing an extra allocation per function load.

So a way to directly pass a *const c_char directly should be added. eg: get_proc_address_raw

[i509VCB]

#1435 has changed glutin to use &CStr for get_proc_address