add support for GL_EXT_debug_marker

This extension is always added to the GL_EXTENSIONS
extension string for the current GL context, regardless
of if it's supported by the h/w driver.

The extension itself will be handled by GLES_trace (eventually),
when GLES_trace is not enabled, it'll result to a no-op.

If the h/w implementation has this extension, we'll call that version
instead of our dummy version.

Change-Id: Ie5dd3387c4d45cd5ed5f03b73bda6045620a96bc
diff --git a/opengl/libs/GLES2/gl2.cpp b/opengl/libs/GLES2/gl2.cpp
index df22b96..79aa3cd 100644
--- a/opengl/libs/GLES2/gl2.cpp
+++ b/opengl/libs/GLES2/gl2.cpp
@@ -110,6 +110,20 @@
 #undef CALL_GL_API
 #undef CALL_GL_API_RETURN
 
+/*
+ * glGetString() is special because we expose some extensions in the wrapper
+ */
+
+extern "C" const GLubyte * __glGetString(GLenum name);
+
+const GLubyte * glGetString(GLenum name)
+{
+    const GLubyte * ret = egl_get_string_for_current_context(name);
+    if (ret == NULL) {
+        ret = __glGetString(name);
+    }
+    return ret;
+}
 
 /*
  * These GL calls are special because they need to EGL to retrieve some