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;
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<div class='path'>path: <a href='/guix/log/?id=9f391b90faca02ca97c5018d6c095ecdaa1a94a7'>root</a>/<a href='/guix/log/gnu?id=9f391b90faca02ca97c5018d6c095ecdaa1a94a7'>gnu</a>/<a href='/guix/log/gnu/installer?id=9f391b90faca02ca97c5018d6c095ecdaa1a94a7'>installer</a>/<a href='/guix/log/gnu/installer/services.scm?id=9f391b90faca02ca97c5018d6c095ecdaa1a94a7'>services.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/gnu/installer/services.scm?id=9f391b90faca02ca97c5018d6c095ecdaa1a94a7&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-08-08 11:54:25 +0200'>2022-08-08</span></td><td><a href='/guix/commit/gnu/installer/services.scm?id=ff9522fb69b9f4a31a5b766029e26dc53a2d1cf8'>installer: Add comments and vertical space to the generated config.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-12-28 15:26:39 -0500'>2021-12-28</span></td><td><a href='/guix/commit/gnu/installer/services.scm?id=0d9d151424ab5823e441f056237819277b8aa072'>installer: Offer 'gpm-service-type' for non-graphical systems.</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>
<tr><td><span title='2021-12-28 15:26:39 -0500'>2021-12-28</span></td><td><a href='/guix/commit/gnu/installer/services.scm?id=2842a42b518227607fed4470b0de1c84e3511beb'>installer: Recommend 'ntp-service-type' for non-graphical systems.</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>
<tr><td><span title='2021-06-30 21:34:26 +0200'>2021-06-30</span></td><td><a href='/guix/commit/gnu/installer/services.scm?id=6f13881f1e92832023caadb3cb51ce393e685e58'>installer: Offer the CUPS printing service.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2021-06-23 03:01:49 +0200'>2021-06-23</span></td><td><a href='/guix/commit/gnu/installer/services.scm?id=ed44e62636873d2b97eff06be70799a0f679bb2b'>installer: Remove unused procedure.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-12-11 19:06:53 +0100'>2020-12-11</span></td><td><a href='/guix/commit/gnu/installer/services.scm?id=d8051557aee9fa252b015ff67cc15681e8540777'>Revert "services: openssh: Warn about 'password-authentication?' default."</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-12-07 12:48:26 +0100'>2020-12-07</span></td><td><a href='/guix/commit/gnu/installer/services.scm?id=aecd2a13cbd8301d0fdeafcacbf69e12cc3f6138'>services: openssh: Warn about 'password-authentication?' default.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-10-13 21:17:40 +0200'>2020-10-13</span></td><td><a href='/guix/commit/gnu/installer/services.scm?id=1197b8b20f4fca4ce03bbc5fa75e18d54e3717c0'>installer: Add Emacs EXWM desktop environment.</a><span class='msg-avail'>...</span></td><td>Jan (janneke) Nieuwenhuizen</td></tr>