Copied from Debian.

From 03a820b173ed1fdef6ff14b4468f5dbc02ff59be Mon Sep 17 00:00:00 2001
From: Heiko Lewin <heiko.lewin@worldiety.de>
Date: Tue, 15 Dec 2020 16:48:19 +0100
Subject: [PATCH] Fix mask usage in image-compositor

[trimmed test case, since not used in Debian build]

---
 src/cairo-image-compositor.c                |   8 ++--

--- cairo-1.16.0.orig/src/cairo-image-compositor.c
+++ cairo-1.16.0/src/cairo-image-compositor.c
@@ -2601,14 +2601,14 @@ _inplace_src_spans (void *abstract_rende
 		    unsigned num_spans)
 {
     cairo_image_span_renderer_t *r = abstract_renderer;
-    uint8_t *m;
+    uint8_t *m, *base = (uint8_t*)pixman_image_get_data(r->mask);
     int x0;
 
     if (num_spans == 0)
 	return CAIRO_STATUS_SUCCESS;
 
     x0 = spans[0].x;
-    m = r->_buf;
+    m = base;
     do {
 	int len = spans[1].x - spans[0].x;
 	if (len >= r->u.composite.run_length && spans[0].coverage == 0xff) {
@@ -2646,7 +2646,7 @@ _inplace_src_spans (void *abstract_rende
 				      spans[0].x, y,
 				      spans[1].x - spans[0].x, h);
 
-	    m = r->_buf;
+	    m = base;
 	    x0 = spans[1].x;
 	} else if (spans[0].coverage == 0x0) {
 	    if (spans[0].x != x0) {
@@ -2675,7 +2675,7 @@ _inplace_src_spans (void *abstract_rende
 #endif
 	    }
 
-	    m = r->_buf;
+	    m = base;
 	    x0 = spans[1].x;
 	} else {
 	    *m++ = spans[0].coverage;
4ea6e8a61'>commit</a><a href='/guix/diff/tests/cpan.scm?id=f16db7fd1bd325375b7ea064166e4314ea6e8a61'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/cpan.scm'>
<input type='hidden' name='id' value='f16db7fd1bd325375b7ea064166e4314ea6e8a61'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=f16db7fd1bd325375b7ea064166e4314ea6e8a61'>root</a>/<a href='/guix/log/tests?id=f16db7fd1bd325375b7ea064166e4314ea6e8a61'>tests</a>/<a href='/guix/log/tests/cpan.scm?id=f16db7fd1bd325375b7ea064166e4314ea6e8a61'>cpan.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/cpan.scm?id=f16db7fd1bd325375b7ea064166e4314ea6e8a61&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2023-06-09 14:19:14 +0200'>2023-06-09</span></td><td><a href='/guix/commit/tests/cpan.scm?id=f69d14d31af8325807d8529fccd3f0b6fd86e342'>tests: Ensure 'cpan' updater test does not access the network.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2023-05-31 23:50:06 +0200'>2023-05-31</span></td><td><a href='/guix/commit/tests/cpan.scm?id=654fcf9971bb01389d577be07c6ec0f68940c743'>tests: Use quasiquoted 'match' patterns for package sexps.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2023-05-31 23:25:25 +0200'>2023-05-31</span></td><td><a href='/guix/commit/tests/cpan.scm?id=a738c1a0c75af8b65ab5cad893e9ac2af6633d29'>import: cpan: Updater provides input list.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2023-05-31 23:25:25 +0200'>2023-05-31</span></td><td><a href='/guix/commit/tests/cpan.scm?id=c4fe4e7eb8ccbe5e8dcbcb8e64fbb3a97a498808'>import: cpan: Represent dependencies as &lt;upstream-input&gt; records.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-10-22 01:46:55 +0200'>2022-10-22</span></td><td><a href='/guix/commit/tests/cpan.scm?id=a3619079f95213c4f983e69210ed12b38fd31022'>Remove now unnecessary uses of (guix grafts).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>