QOpenGLTextureHelper: explain why we use GetProcAddress on Windows / DesktopGL

In the future, we might want to refactor this code to inconditionally
use the common Desktop / ES2 subset (like the rest of Qt does),
and then resolve only the functions actually available at runtime.

For now, state why we're doing that on Windows.

Change-Id: Ic21035bcd88ddc1d9274fd90a146c2824d783b25
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>

src/gui/opengl/qopengltexturehelper.cpp

@@ -139,6 +139,9 @@ QOpenGLTextureHelper::QOpenGLTextureHelper(QOpenGLContext *context)
     }
 #endif
 
+    // wglGetProcAddress should not be used to (and indeed will not) load OpenGL <= 1.1 functions.
+    // Hence, we resolve them "the hard way"
+
 #if defined(Q_OS_WIN) && !defined(QT_OPENGL_ES_2)
     HMODULE handle = GetModuleHandleA("opengl32.dll");