filters: rewrite colorize in c

Since its introduction, we had multiple issues with the colorize awk
script with regard to non-GNU awk compatibility.

Also, this script is standalone and the color theme must be hard coded
into it. Reading from an external configuration file (aerc's styleset)
from a non-GNU awk is close to impossible (and even far from trivial
with GNU awk).

Rewrite the builtin colorize filter in C to allow getting the color
theme from aerc's active styleset. The theme is configured using the
existing styleset syntax and attributes under a separate [viewer]
section (see examples and man page).

Export the active styleset file path to AERC_STYLESET env var when
invoking the filter command so that colorize can access it and use it.

I have tested compilation (with clang-analyzer and gcc -fanalyzer) and
basic operation on FreeBSD, Fedora (glibc) and Alpine (muslibc). More
tests would probably be required on MacOSX and older Linux distros.
I also added test vectors to give some confidence that this works as
expected. The execution with these vectors passed valgrind
--leak-check=full without errors.

NB: the default theme has changed to be more minimal. Sample stylesets
have more colorful examples. The awk -v theme=xxx option is no longer
supported.

  usage: colorize [-h] [-s FILE] [-f FILE]

  options:
    -h       show this help message
    -s FILE  use styleset file (default $AERC_STYLESET)
    -f FILE  read from filename (default stdin)

Signed-off-by: Robin Jarry <robin@jarry.cc>
Tested-by: Bence Ferdinandy <bence@ferdinandy.com>
Acked-by: Moritz Poldrack <moritz@poldrack.dev>

2 files changed, 33 insertions, 6 deletions

diff --git a/doc/aerc-config.5.scd b/doc/aerc-config.5.scd
index cea7d2e7..cc0a5f80 100644
--- a/doc/aerc-config.5.scd
+++ b/doc/aerc-config.5.scd
@@ -653,11 +653,8 @@ _text/plain_
 	text/plain=colorize
 	```
 
-	The built-in _colorize_ filter supports alternative themes:
-
-	```
-	text/plain=colorize -v theme=solarized
-	```
+	The built-in _colorize_ filter can be configured in the *[viewer]*
+	section of styleset files. See *aerc-stylesets*(7).
 
 	Wrap long lines at 100 characters, while not messing up nested quotes.
 	Handles format=flowed emails properly:
@@ -766,7 +763,7 @@ _image/\*_
 	```
 
 See the wiki at https://man.sr.ht/~rjarry/aerc/ for more examples and possible
-customizations of the built-in filters (e.g. colors of _colorize_).
+customizations of the built-in filters.
 
 # OPENERS
 
diff --git a/doc/aerc-stylesets.7.scd b/doc/aerc-stylesets.7.scd
index 1303787c..eaedb4cb 100644
--- a/doc/aerc-stylesets.7.scd
+++ b/doc/aerc-stylesets.7.scd
@@ -136,6 +136,36 @@ styling.
 |  *selector_chooser*
 :  The item chooser in a selector ui element.
 
+These next style objects only affect the built-in *colorize* filter and must be
+declared under a *[viewer]* section of the styleset file.
+
+[[ *Style Object*
+:[ *Description*
+|  *url*
+:  URLs.
+|  *header*
+:  RFC-822-like header names.
+|  *signature*
+:  Email signatures.
+|  *diff_meta*
+:  Patch diff meta lines.
+|  *diff_chunk*
+:  Patch diff chunks.
+|  *diff_add*
+:  Patch diff added lines.
+|  *diff_del*
+:  Patch diff deleted lines.
+|  *quote_1*
+:  First level quoted text.
+|  *quote_2*
+:  Second level quoted text.
+|  *quote_3*
+:  Third level quoted text.
+|  *quote_4*
+:  Fourth level quoted text.
+|  *quote_x*
+:  Above fourth level quoted text.
+
 ## FNMATCH STYLE WILDCARD MATCHING
 
 The styleset configuration can be made simpler by using the fnmatch