Fix font options leak in gstate

cairo_gstate_t contains a cairo_font_options_t. Since commit 67eeed44,
this can contain an extra allocation for a custom palette. Since commit
edf9497c3a, this contains an extra allocation for a string. Before these
commit, font options could just be dropped, but now they need to be
freed.

This commit makes _cairo_gstate_fini() finish the font options to free
the memory allocation.

The new test was run via "valgrind --leak-check=full ./cairo-test-suite
-f leaks-set-scaled-font". The following reported leak goes away thanks
to this commit:

 1,040 bytes in 26 blocks are definitely lost in loss record 6 of 12
    at 0x48407B4: malloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
    by 0x4886C62: _cairo_font_options_init_copy (cairo-font-options.c:105)
    by 0x488C029: _cairo_gstate_set_font_options (cairo-gstate.c:1757)
    by 0x48841D7: _cairo_default_context_set_scaled_font (cairo-default-context.c:1310)
    by 0x490809A: cairo_set_scaled_font (cairo.c:3318)
    by 0x15BF1F: leaks_set_scaled_font (leaks.c:45)
    by 0x129EF0: cairo_test_for_target (cairo-test.c:938)
    by 0x12B37F: _cairo_test_context_run_for_target (cairo-test.c:1545)
    by 0x12C385: _cairo_test_runner_draw (cairo-test-runner.c:258)
    by 0x12DEB5: main (cairo-test-runner.c:962)

Fixes: https://gitlab.freedesktop.org/cairo/cairo/-/issues/795
Signed-off-by: Uli Schlachter <psychon@znc.in>

src/cairo-gstate.c

@@ -193,6 +193,7 @@ void
 _cairo_gstate_fini (cairo_gstate_t *gstate)
 {
     _cairo_stroke_style_fini (&gstate->stroke_style);
+    _cairo_font_options_fini (&gstate->font_options);
 
     cairo_font_face_destroy (gstate->font_face);
     gstate->font_face = NULL;

test/leaks.c

@@ -0,0 +1,61 @@
+/*
+ * Copyright © 2023 Uli Schlachter
+ *
+ * Permission is hereby granted, free of charge, to any person
+ * obtaining a copy of this software and associated documentation
+ * files (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use, copy,
+ * modify, merge, publish, distribute, sublicense, and/or sell copies
+ * of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+ * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ * Author: Uli Schlachter <psychon@znc.in>
+ */
+
+#include "cairo-test.h"
+
+
+// Once upon a time, _cairo_gstate_fini leaked font options
+static cairo_test_status_t
+leaks_set_scaled_font (cairo_t *cr, int width, int height)
+{
+    cairo_font_options_t *opt;
+    cairo_matrix_t matrix;
+    cairo_scaled_font_t *font;
+
+    cairo_matrix_init_identity (&matrix);
+
+    opt = cairo_font_options_create ();
+    cairo_font_options_set_custom_palette_color (opt, 0, 1, 1, 1, 1);
+
+    font = cairo_scaled_font_create (cairo_get_font_face (cr), &matrix, &matrix, opt);
+
+    cairo_set_scaled_font (cr, font);
+
+    cairo_font_options_destroy (opt);
+    cairo_scaled_font_destroy (font);
+
+    // Fill the output so that the same ref image works for everying
+    cairo_paint (cr);
+
+    return CAIRO_TEST_SUCCESS;
+}
+
+CAIRO_TEST (leaks_set_scaled_font,
+	    "Regression test for font options memory leak in cairo_set_scaled_font",
+	    "leak", /* keywords */
+	    NULL, /* requirements */
+	    1, 1,
+	    NULL, leaks_set_scaled_font)

test/meson.build

@@ -190,6 +190,7 @@ test_sources = [
   'large-source.c',
   'large-source-roi.c',
   'large-twin-antialias-mixed.c',
+  'leaks.c',
   'leaky-dash.c',
   'leaky-dashed-rectangle.c',
   'leaky-dashed-stroke.c',