new glx: implement blur

This is barely tested, so bugs and/or performance problems to be
expected.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/gl/glx.c

@@ -240,6 +240,11 @@ static backend_t *glx_init(session_t *ps) {
 		goto end;
 	}
 
+	if (glXGetConfig(ps->dpy, pvis, GLX_STENCIL_SIZE, &value) || !value) {
+		log_error("Root visual lacks stencil buffer.");
+		goto end;
+	}
+
 	if (glXGetConfig(ps->dpy, pvis, GLX_DOUBLEBUFFER, &value) || !value) {
 		log_error("Root visual is not a double buffered GL visual.");
 		goto end;