path: start special espansion with ~~name instead of ~name

Since commit 7381db60, strings like "~desktop/" were expanded as
platform-specific paths by mpv. Apparently this similarity to standard
Unix shell expansion caused confusion, so change it to "~~desktop/". The
shell doesn't expand this, so it should be better.

options/path.c

@@ -182,7 +182,7 @@ char *mp_get_user_path(void *talloc_ctx, struct mpv_global *global,
                 res = mp_find_config_file(talloc_ctx, global, rest0);
             } else if (bstr_equals0(prefix, "")) {
                 res = mp_path_join(talloc_ctx, bstr0(getenv("HOME")), rest);
-            } else {
+            } else if (bstr_eatstart0(&prefix, "~")) {
                 char type[80];
                 snprintf(type, sizeof(type), "%.*s", BSTR_P(prefix));
                 const char *p = mp_get_platform_path(talloc_ctx, type);