Always use GL_Cull to change cull state

Manually changing cull state can cause later GL_Cull calls to not change
the cull state.

code/renderergl2/tr_fbo.c

@@ -748,7 +748,7 @@ void FBO_BlitFromTexture(struct image_s *src, ivec4_t inSrcBox, vec2_t inSrcTexS
 
 	Mat4Ortho(0, width, height, 0, 0, 1, projection);
 
-	qglDisable( GL_CULL_FACE );
+	GL_Cull( CT_TWO_SIDED );
 
 	GL_BindToTMU(src, TB_COLORMAP);