[Fontconfig] Changing to hintslight globally by default + hintfull for otfs

[Nikolaus Waxweiler]

> Scrapping properties seems like a bad idea.

Inside a minor version definitely. This was more a thought for a 
hypothetical fontconfig 3.0.0. This would also, as far as I can see, 
need a new load flag in FT to indicate the desired hinting behavior.

> Re-defining what existing properties actually do in regards to
> rendering seems like the way to go. Do anything possible via default
> configuration and use the current hintstyles to do what's desired.

You mean, do something like x-and-y-if-supported-native-hinter = 
hintfull, y-only-native-hinter = medium, y-only-autohinter = slight? 
That might work just as well. In case that's what it already means, it 
needs to be better documented.

> Also, rendering is subjective.

True, but only partly. People have strong opinions about what looks best 
to them, but this is more about jarring differences in hinting behavior 
and output quality. The DejaVu family vs. Cantarell in a standard Fedora 
installation is a case in point for different hinting behavior, but 
there is more.

What prompted me initially to propose a global hintslight was a long 
thread in a comments section of an article about a new distribution 
release. Somebody complained that the fonts in the main screenshots 
looked low-quality (inconsistent kerning, ...) like in many other 
distributions. I presume that most distributions use the defaults, so I 
figured, why not go to the source.

Most fonts in the wild are TrueType (.ttf) fonts. Hinting them well 
takes more time than most open-source efforts can muster and using 
ttfautohint to do the job is basically using FreeType's autohinter. On 
top of that, FreeType's subpixel hinting code for ClearType support is 
still experimental and disabled by default, so many fonts on the web 
will not really render as intended!

Hintfull as the global default will show users less-than-ideal rendering 
results out of the box. FreeType's autohinter (hintslight) in contrast 
hints most fonts I encountered quite well, regardless of font format. So 
I propose hintslight as the default with a note to users that still want 
their DejaVu razor sharp :)