OGL: Use a fixed VAO for attributeless rendering.

Instead of abusing whatever VAO is previously bound, which might have
enabled arrays.

Only used in one instance currently, which fixes a crash with older
NVIDIA drivers.

Source/Core/VideoBackends/OGL/PostProcessing.cpp

@@ -72,6 +72,8 @@ void OpenGLPostProcessing::BlitFromTexture(TargetRectangle src, TargetRectangle
 
 	if (m_attribute_workaround)
 		glBindVertexArray(m_attribute_vao);
+	else
+		OpenGL_BindAttributelessVAO();
 
 	m_shader.Bind();