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;
14b9cd871c7a8d'>commit</a><a href='/guix/diff/gnu/machine.scm?id=e7cdba61cf67abb5844c342a7614b9cd871c7a8d'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/machine.scm'>
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<div class='path'>path: <a href='/guix/log/?id=e7cdba61cf67abb5844c342a7614b9cd871c7a8d'>root</a>/<a href='/guix/log/gnu?id=e7cdba61cf67abb5844c342a7614b9cd871c7a8d'>gnu</a>/<a href='/guix/log/gnu/machine.scm?id=e7cdba61cf67abb5844c342a7614b9cd871c7a8d'>machine.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/machine.scm?id=e7cdba61cf67abb5844c342a7614b9cd871c7a8d&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2019-12-08 00:47:26 +0100'>2019-12-08</span></td><td><a href='/guix/commit/gnu/machine.scm?id=a1d7920861a8d6c6774149eaf939eb2636942090'>machine: Remove unnecessary record self-referencing bindings.</a><span class='msg-avail'>...<span class='msg-tooltip'>'this-machine' and 'this-machine-ssh-configuration' were useless given
that there are no thunked fields.

* gnu/machine.scm (&lt;machine&gt;)[this-machine]: Remove.
* gnu/machine/ssh.scm (&lt;machine-ssh-configuration&gt;)
[this-machine-ssh-configuration]: Remove.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-12-07 00:59:56 +0100'>2019-12-07</span></td><td><a href='/guix/commit/gnu/machine.scm?id=eaabc5e87f4e48d7bce88ca231f5fc2d554ca3d6'>machine: Add provenance tracking to each machine operating system.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/machine.scm (&lt;machine&gt;): Rename accessor to
'%machine-operating-system'.
(machine-operating-system): New procedure.
* doc/guix.texi (Service Reference): Mention it.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-08-15 07:43:09 -0400'>2019-08-15</span></td><td><a href='/guix/commit/gnu/machine.scm?id=9c70c460a05b2bc60f3f3602f0a2dba0f79ce86c'>machine: Implement 'roll-back-machine'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/machine.scm (roll-back-machine, &amp;deploy-error, deploy-error?)
(deploy-error-should-roll-back)
(deploy-error-captured-args): New variable.
* gnu/machine/ssh.scm (roll-back-managed-host): New variable.
* guix/scripts/deploy.scm (guix-deploy): Roll-back systems when a
deployment fails.
</span></span></td><td>Jakob L. Kreuze</td></tr>
<tr><td><span title='2019-08-06 15:48:51 -0400'>2019-08-06</span></td><td><a href='/guix/commit/gnu/machine.scm?id=d97ce20400b5d2967bfbc0e9fbfb99bbb2daed4f'>machine: Rename 'system' field.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/machine.scm (machine-system): Delete variable.
(machine-operating-system): New variable.
All callers changed.
* doc/guix.texi (Invoking guix deploy): Use the
'machine-operating-system' accessor rather than 'machine-system'.
</span></span></td><td>Jakob L. Kreuze</td></tr>
<tr><td><span title='2019-07-06 02:10:04 -0400'>2019-07-06</span></td><td><a href='/guix/commit/gnu/machine.scm?id=fa9edf09e992db7510c7471486dffc93e1e707e5'>gnu: Add machine type for deployment specifications.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/machine.scm: New file.
* gnu/machine/ssh.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
</span></span></td><td>Jakob L. Kreuze</td></tr>