Move GrGLInterface function pointers into a nested struct

src/gpu/gl/GrGLUtil.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLUtil_DEFINED
 #define GrGLUtil_DEFINED
 
@@ skipping 136 matching lines @@
     do {                                                        \
         GR_GL_CALL_NOERRCHECK(IFACE, X);                        \
         GR_GL_CHECK_ERROR_IMPL(IFACE, X);                       \
     } while (false)
 
 // Variant of above that always skips the error check. This is useful when
 // the caller wants to do its own glGetError() call and examine the error value.
 #define GR_GL_CALL_NOERRCHECK(IFACE, X)                         \
     do {                                                        \
         GR_GL_CALLBACK_IMPL(IFACE);                             \
-        (IFACE)->f##X;                                          \
+        (IFACE)->fFunctions.f##X;                               \
         GR_GL_LOG_CALLS_IMPL(X);                                \
     } while (false)
 
 // same as GR_GL_CALL but stores the return value of the gl call in RET
 #define GR_GL_CALL_RET(IFACE, RET, X)                           \
     do {                                                        \
         GR_GL_CALL_RET_NOERRCHECK(IFACE, RET, X);               \
         GR_GL_CHECK_ERROR_IMPL(IFACE, X);                       \
     } while (false)
 
 // same as GR_GL_CALL_RET but always skips the error check.
 #define GR_GL_CALL_RET_NOERRCHECK(IFACE, RET, X)                \
     do {                                                        \
         GR_GL_CALLBACK_IMPL(IFACE);                             \
-        (RET) = (IFACE)->f##X;                                  \
+        (RET) = (IFACE)->fFunctions.f##X;                       \
         GR_GL_LOG_CALLS_IMPL(X);                                \
     } while (false)
 
 // call glGetError without doing a redundant error check or logging.
-#define GR_GL_GET_ERROR(IFACE) (IFACE)->fGetError()
+#define GR_GL_GET_ERROR(IFACE) (IFACE)->fFunctions.fGetError()
 
 #endif