XML line numbers above 65535 are reported as 65535

[marnanel]

Describe the bug
All XML line numbers beyond 65535 are reported in warnings and errors as being line 65535.

To Reproduce
Steps to reproduce the behavior:

1. Create an enormous sif file, such as this one: ginormous.zip . This contains a 65535-line comment, before the line which imports wheel.png.
2. Rename wheel.png to something else, so that rendering ginormous.sif will cause a warning.
3. Render ginormous.sif (either from the commandline or from Studio).
4. Note the warning: synfig(261538) [20:11:50] warning: ginormous.sif:<string>:65535: Layer 'import' rejected value for parameter 'filename'
5. Note also that the import layer is not at line 65535:

$ grep -n wheel.png ginormous.sif
65600:        <layer type="import" active="true" exclude_from_rendering="false" version="0.1" desc="wheel.png">

Expected behavior
The warning should show the correct line, thus:

synfig(261538) [20:11:50] warning: ginormous.sif:<string>:65600: Layer 'import' rejected value for parameter 'filename'

System information:

• OS: Debian
• OS version: 12.2
• Commit ID: 69f4ea7

Additional comments
This is an upstream issue with libxml2. (Presumably in the past someone at libxml2 decided that 64K was enough for anybody.)

The fix: Before each xmlpp::DomParser is used to parse_stream(), call set_parser_options with the flag [XML_PARSE_BIG_LINES](https://gnome.pages.gitlab.gnome.org/libxml2/devhelp/libxml2-parser.html#xmlParserOption).

DomParsers are created in six places in Synfig. I can make a PR to fix this, but I don't know my way around your code very well, so I might need to ask a few questions.

[ice0]

Hi!

I can make a PR to fix this, but I don't know my way around your code very well, so I might need to ask a few questions.

Sure, that will be great.