QOpenGLTextureHelper (Windows): Don't load desktop DLL when GLES2 is used

GL functions should come from the context, not the desktop GL lib, when
Qt is configured for OpenGL ES 2 (e.g. ANGLE).

Change-Id: I794e1d5989ac72d2e98070d20e91f9c2c4cb7183
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>