When printing, behave the same way as when parsing:
Magically skip a container node if there is one - just like the
parser magically creates a container node to hold all the nodes
it parses.
We don't want to return a GFile because GFile can't handle can't deal
with data: urls.
That makes the code a bit more complicated that doesn't deal with those
URLs, but it makes the other code actually work.
GtkCssImageUrl also now decodes data urls immediately instead of only at
the first load. So don't use data urls if you care about performance.
Instead of encoding the raw data, encode the full image to a PNG.
And instead of stuffing that encoding into a string, use a full
data: url.
And then remove the width and height properties, because they're now
implicitly included in the data.
And then change the parser to match.
And because the parser now parses regular urls on top of data: urls, we
can now load any random file.
Instead of the previous approach using GVariant, this new approach uses
human-readable text files as the serialization format for render nodes.
The format is a custom one, but it is inspired by QML and conforms to
the CSS syntax. Because of that, we can use the CSS machinery from GTK
to parse it, and in particular share code to parse properties that GTK's
CSS machinery also supports, such as colors.
This commit breaks all existing usages of node files - such as the
testsuite and various test tools - they will be fixed in further
commits.
