Fix bilinear filter computation in wide pipeline

pixman 0.38.x has a regression that causes incorrect rendering in some
circumstances. This can be triggered by the use of cairo with
CAIRO_OPERATOR_SATURATE and subpixel positioning, and causes OpenSlide
to produce incorrect output.

This patch is a cherry-pick of [1], rebasing it on top of `c8s`.

It backports commit 8256c235d9b3 ("Fix bilinear filter computation in
wide pipeline") [2] from pixman 0.40.0 to fix the mentioned regression.

At the moment of writing this, pixman's version is:

 - Fedora: 0.42.2
 - CentOS Stream 9: 0.40.0
 - CentOS Stream 8: 0.38.4

Therefore, CentOS Stream 8 needs to be patched.

A reproducer can be found in the original bug report [3].

[1] https://gitlab.com/redhat/centos-stream/rpms/pixman/-/merge_requests/2
[2] 8256c235d9
[3] https://bugzilla.redhat.com/show_bug.cgi?id=2124013
Resolves: https://issues.redhat.com/browse/RHEL-3061

0001-Fix-bilinear-filter-computation-in-wide-pipeline.patch

@@ -0,0 +1,84 @@
+From 8256c235d9b3854d039242356905eca854a890ba Mon Sep 17 00:00:00 2001
+From: Basile Clement <basile-pixman@clement.pm>
+Date: Tue, 9 Apr 2019 23:16:13 +0200
+Subject: [PATCH] Fix bilinear filter computation in wide pipeline
+
+The recently introduced wide pipeline for filters has a typo which
+causes it to improperly compute bilinear interpolation positions,
+causing various glitches when enabled.
+
+This patch uses the proper computation for bilinear interpolation in the
+wide pipeline.  It also makes related `if` statements conformant to the
+CODING_STYLE:
+
+* If a substatement spans multiple lines, then there must be braces
+  around it.
+
+* If one substatement of an if statement has braces, then the other
+  must too.
+
+Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
+---
+ pixman/pixman-bits-image.c | 9 +++++++++
+ pixman/pixman-inlines.h    | 2 +-
+ 2 files changed, 10 insertions(+), 1 deletion(-)
+
+diff --git a/pixman/pixman-bits-image.c b/pixman/pixman-bits-image.c
+index 564789e..7bc2ba8 100644
+--- a/pixman/pixman-bits-image.c
++++ b/pixman/pixman-bits-image.c
+@@ -432,29 +432,38 @@ bits_image_fetch_pixel_filtered (bits_image_t  *image,
+ 
+     case PIXMAN_FILTER_CONVOLUTION:
+ 	if (wide)
++	{
+ 	    bits_image_fetch_pixel_convolution (image, x, y,
+ 						get_pixel, out,
+ 						accum_float,
+ 						reduce_float);
++	}
+ 	else
++	{
+ 	    bits_image_fetch_pixel_convolution (image, x, y,
+ 						get_pixel, out,
+ 						accum_32, reduce_32);
++	}
+ 	break;
+ 
+     case PIXMAN_FILTER_SEPARABLE_CONVOLUTION:
+ 	if (wide)
++	{
+ 	    bits_image_fetch_pixel_separable_convolution (image, x, y,
+ 							  get_pixel, out,
+ 							  accum_float,
+ 							  reduce_float);
++	}
+ 	else
++	{
+ 	    bits_image_fetch_pixel_separable_convolution (image, x, y,
+ 							  get_pixel, out,
+ 							  accum_32, reduce_32);
++	}
+         break;
+ 
+     default:
++	assert (0);
+         break;
+     }
+ }
+diff --git a/pixman/pixman-inlines.h b/pixman/pixman-inlines.h
+index 332e208..f785910 100644
+--- a/pixman/pixman-inlines.h
++++ b/pixman/pixman-inlines.h
+@@ -231,7 +231,7 @@ bilinear_interpolation_float (argb_t tl, argb_t tr,
+     argb_t r;
+ 
+     distxy = distx * disty;
+-    distxiy = distx - (1.f - distxy);
++    distxiy = distx * (1.f - disty);
+     distixy = (1.f - distx) * disty;
+     distixiy = (1.f - distx) * (1.f - disty);
+ 
+-- 
+2.37.1
+

pixman.spec

@@ -3,7 +3,7 @@
 
 Name:           pixman
 Version:        0.38.4
-Release:        2%{?dist}
+Release:        3%{?dist}
 Summary:        Pixel manipulation library
 
 Group:          System Environment/Libraries
@@ -17,6 +17,7 @@ Source0:        https://xorg.freedesktop.org/archive/individual/lib/%{name}-%{ve
 Source1:        make-pixman-snapshot.sh
 
 Patch0: 0001-Initialize-temporary-buffers-in-general_composite_re.patch
+Patch1: 0001-Fix-bilinear-filter-computation-in-wide-pipeline.patch
 
 BuildRequires:  automake autoconf libtool
 BuildRequires:  gcc
@@ -68,6 +69,9 @@ make check %{?_smp_mflags} V=1
 %{_libdir}/pkgconfig/pixman-1.pc
 
 %changelog
+* Sat Sep 03 2022 Benjamin Gilbert <bgilbert@backtick.net> - 0.38.4-3
+- Fix bilinear filter computation in wide pipeline
+
 * Tue Feb 22 2022 Adam Jackson <ajax@redhat.com> - 0.38.4-2
 - Backport the pixman part of cairo CVE-2020-35492