Fix OpenGL function prototype calling conventions

Fix OpenGL function prototype calling conventions

OpenGL doesn't use the default function calling convention
on all platforms, which is why gl.h defines the macros
GL_APICALL and GL_APIENTRY for applications to use when
defining their own OpenGL function prototypes. Not using
these may cause a crash if an OpenGL function is called
with the wrong calling convention.

Affects:
KHR-GL46.transform_feedback.*

Components: OpenGL

VK-GL-CTS issue: 882

Change-Id: I8766262b32a04b8eff8e110e1f116c2f915274e9