Fix SetFontFace method to be inline with LoadFontFace

[harture]

No description provided.

[fogleman]

I'd like to merge this but I need to be careful with changing API behavior. Thoughts?

[remeh]

No solution for the API but I just want to +1 the fix, I've been bitten by this recently.

I was loading each time the font using dc.LoadFontface(string, float64) when I needed to render in a given size (at this time performances were not in the scope).
Later, for performances improvements, I've added a font cache: caching the font with
gg.LoadFontFace(string, float64) and using the font with dc.SetFontFace(font.Face). I've been surprised that the rendering result was not quite the same.

[mufty]

Yes this is fixing/unifying the SetFontFace method. I'm also trying to do the same thing as @remeh did with different results. Thus we have a fork at the moment with this PR included and it seems to work fine.

However one thing I don't understand is what the hell are these numbers in LoadFontface supposed to mean? What's " * 72 / 96"? 72 of what? and 96 of what?

I had a similar issue with font sizing in the past. Working with a different library in a different language but the root cause is the same I think. The issue was that the font description almost never has correct values for width and height of the font. What I had to do is to use glyphs to get a real text size just before "rendering" it. This is significantly slower because I had to basically render the whole text and get a real height after that but it works every time. You can remove these magic numbers doing that completely at the cost of speed tho.

[kirides]

iirc, 72 is Pixels per Inch and 96 is the assumed DPI for any monitor.

Though i only know this because of working with C# Winforms & WPF. Winforms assumes everything is in PPI (72px) and WPF uses proper N * (PPI / DPI)

This hit me, because i was hosting a winforms control inside WPF and the fontsizes didn't match.