Add GetProcAddress to OpenGL control interface

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Note: This header should be compilable as C.
 
 #ifndef MOJO_PUBLIC_C_GPU_MGL_MGL_H_
 #define MOJO_PUBLIC_C_GPU_MGL_MGL_H_
 
 #include <stdint.h>
@@ skipping 37 matching lines @@
 void MGLDestroyContext(MGLContext context);
 
 // Makes |context| the current MGLContext for the calling thread. Calling with
 // MGL_NO_CONTEXT clears the current context.
 void MGLMakeCurrent(MGLContext context);
 
 // Returns the currently bound context for the calling thread or MGL_NO_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
 
 #endif  // MOJO_PUBLIC_C_GPU_MGL_MGL_H_