rules: skip heading UTF-8 encoded BOM (U+FEFF)

Leading BOM is legal and is used as a signature — an indication that
an otherwise unmarked text file is in UTF-8.
See: https://www.unicode.org/faq/utf_bom.html#bom5 for further details.

src/xkbcomp/rules.c

@@ -1099,6 +1099,10 @@ read_rules_file(struct xkb_context *ctx,
 
     scanner_init(&scanner, matcher->ctx, string, size, path, NULL);
 
+    /* Skip UTF-8 encoded BOM (U+FEFF) */
+    /* See: https://www.unicode.org/faq/utf_bom.html#bom5 */
+    scanner_str(&scanner, "\xef\xbb\xbf", 3);
+
     ret = matcher_match(matcher, &scanner, include_depth, string, size, path);
 
     unmap_file(string, size);

test/data/rules/utf-8_with_bom

@@ -0,0 +1,22 @@
+// NOTE: this file is encoded in UTF-8 with a leading BOM (U+FEFF)
+! model         = keycodes
+  my_model      = my_keycodes
+  *             = default_keycodes
+
+! layout        variant    = symbols
+  my_layout     my_variant = my_symbols+extra_variant
+
+! layout        = symbols
+  my_layout     = my_symbols
+  *             = default_symbols
+
+! model         = types
+  my_model      = my_types
+  *             = default_types
+
+! model         = compat
+  my_model      = my_compat
+  *             = default_compat
+
+! option        = compat
+  my_option     = |some:compat

test/rules-file.c

@@ -94,6 +94,18 @@ main(int argc, char *argv[])
     ctx = test_get_context(0);
     assert(ctx);
 
+    struct test_data test_utf_8_with_bom = {
+        .rules = "utf-8_with_bom",
+
+        .model = "my_model", .layout = "my_layout", .variant = "my_variant",
+        .options = "my_option",
+
+        .keycodes = "my_keycodes", .types = "my_types",
+        .compat = "my_compat|some:compat",
+        .symbols = "my_symbols+extra_variant",
+    };
+    assert(test_rules(ctx, &test_utf_8_with_bom));
+
     struct test_data test1 = {
         .rules = "simple",