Issue 6210: Subpixel font rendering system settings should only apply…

… to GIMP not text layer rendering in image

Patch was merged n Cairo today, wouhou!
https://gitlab.freedesktop.org/cairo/cairo/-/merge_requests/114
If not mistaken, the fix should then appear in Cairo 1.17.6 or 1.18.0
(whatever comes next). As we are obviously not going to bump the Cairo
dependency so early, let's just add the patch, at the very least to be
used for our official builds. Also this way, we won't forget about this
issue in the future when we need to bump Cairo.

build/patches/cairo-mr114-gimp-issue-6210.patch

@@ -0,0 +1,44 @@
+From 4f4d89506f58a64b4829b1bb239bab9e46d63727 Mon Sep 17 00:00:00 2001
+From: Jehan <jehan@girinstud.io>
+Date: Tue, 19 Jan 2021 12:42:31 +0100
+Subject: [PATCH] src: do not override explicitly requested grayscale
+ antialiasing.
+
+If CAIRO_ANTIALIAS_DEFAULT is selected and system is set to subpixel, it
+is perfectly normal to switch to CAIRO_ANTIALIAS_SUBPIXEL.
+But when the calling application specifically requested
+CAIRO_ANTIALIAS_GRAY then Cairo should honor the request.
+
+This is an issue we have had for years in GIMP, where text on images
+would render differently depending on the system the file is opened on.
+This is obviously not right as a graphics work should be system
+independant and allow the creator to decide if one wants a text to have
+grayscale or subpixel rendering (a settings which would stick when
+sharing the work file). Cairo should not override this.
+The CAIRO_ANTIALIAS_DEFAULT settings exists exactly for this (when we
+want Cairo to choose for us, in a system-dependant way); other settings
+are when we need system independance.
+
+Thanks to Adam Fontenot for initial investigations and tests on this and
+other contributors before this.
+---
+ src/cairo-ft-font.c | 3 +--
+ 1 file changed, 1 insertion(+), 2 deletions(-)
+
+diff --git a/src/cairo-ft-font.c b/src/cairo-ft-font.c
+index d2221edec..1f5befec3 100644
+--- a/src/cairo-ft-font.c
++++ b/src/cairo-ft-font.c
+@@ -1945,8 +1945,7 @@ _cairo_ft_options_merge (cairo_ft_options_t *options,
+     }
+ 
+     if (other->base.antialias == CAIRO_ANTIALIAS_SUBPIXEL &&
+-	(options->base.antialias == CAIRO_ANTIALIAS_DEFAULT ||
+-	 options->base.antialias == CAIRO_ANTIALIAS_GRAY)) {
++	options->base.antialias == CAIRO_ANTIALIAS_DEFAULT) {
+ 	options->base.antialias = CAIRO_ANTIALIAS_SUBPIXEL;
+ 	options->base.subpixel_order = other->base.subpixel_order;
+     }
+-- 
+GitLab
+