GL: Check if occlusion query functions are found.

src/renderer_gl.cpp

@@ -1824,6 +1824,13 @@ namespace bgfx { namespace gl
 				|| s_extension[Extension::NV_occlusion_query         ].m_supported
 				;
 
+			m_occlusionQuerySupport &= true
+				&& NULL != glGenQueries
+				&& NULL != glDeleteQueries
+				&& NULL != glBeginQuery
+				&& NULL != glEndQuery
+				;
+
 			g_caps.supported |= m_occlusionQuerySupport
 				? BGFX_CAPS_OCCLUSION_QUERY
 				: 0