Add GetProcAddress to OpenGL control interface

mojo/public/c/gpu/MGL/mgl.h

Index: mojo/public/c/gpu/MGL/mgl.h
diff --git a/mojo/public/c/gpu/MGL/mgl.h b/mojo/public/c/gpu/MGL/mgl.h
index 2633976ddbdcc82f3cc8147df3e14cd8e036320f..a74bc861c404fb4e0c1b41107eb48729bb3b16e0 100644
--- a/mojo/public/c/gpu/MGL/mgl.h
+++ b/mojo/public/c/gpu/MGL/mgl.h
@@ -55,6 +55,10 @@ void MGLMakeCurrent(MGLContext context);
 // if there is none.
 MGLContext MGLGetCurrentContext(void);
 
+typedef void (*MGLFunctionPointerType)(void);

[jamesr, 2015/09/21 19:38:50]

this should go in mgl_types.h and have a bit of guidance about how to handle the
value (i.e. cast to appropriate function type).  do we know what calling
convention we want people to use?

[Petr Hosek, 2015/09/22 20:06:54]

I've decided to use `void *` instead which is more akin to other
`GetProcAddress` implementations do, but I'm fine to change it back to a custom
type if you prefer.

+
+MGLFunctionPointerType MGLGetProcAddress(const char* procname);

[jamesr, 2015/09/21 19:38:50]

this should be documented to say what it does, what the preconditions are and
what the restrictions on the returned value are. i think we should say that this
returns null for functions that the implementation does not support, is
independent of the currently current context, returns functions usable in any
context that advertise the corresponding extension in their GL_EXTENSIONS string
and that this only returns GL functions (i.e. it doesn't advertise MGL
functions).

[Petr Hosek, 2015/09/22 20:06:54]

Done.

+
 #ifdef __cplusplus
 }  // extern "C"
 #endif