feat: truetype font features

[erics118]

Adds support for TrueType font features

You can use OpenType font features too, but you need to go through the docs, or <CoreText/SFNTLayoutTypes.h> to find the corresponding TrueType codes.

For example, the C code is:

int feat_name = kNumberSpacingType; // = 6
int feat_value = kMonospacedNumbersSelector; // = 0

CFNumberRef name = CFNumberCreate(default_allocator, kCFNumberIntType, &feat_name);
CFNumberRef value = CFNumberCreate(default_allocator, kCFNumberIntType, &feat_value);

descriptor = CTFontDescriptorCreateCopyWithFeature(descriptor, name, value);

The corresponding way to do this now is:

# one feature
sketchybar --set item label.font.features="6:0"
# multiple features
sketchybar --set item label.font.features="6:0,37:1"

I'm not sure if there is a nicer way of doing this though

Closes #670
In the above issue, the corresponding features string would be "14:4" (kTypographicExtras, kSlashedZeroOnSelector)

Related: #187

[FelixKratz]

I believe this causes a memory leak, because it copies the existing descriptor and then overwrites the pointer.

[FelixKratz]

What we could think about would be to accept both the format you introduced, which is the „low-level“ syntax along with some common features which we give a sensible selector name and internally do the mapping for. What do you think?

[erics118]

I hesitate to use our own namings of these features, as these selectors are very standardized.

What about just manually creating a mapping from opentype features to truetype features? It would just include the most common few features, as there are way too many. This way, users could use some of the opentype names (as i believe opentype is much more prevalent outside of apple)

ie, some sort of mapping like this:

struct {
    char opentype_tag[5];
    int truetype_feature;
    int truetype_selector;
};
// an array of:
{"pnum", kNumberSpacingType, kProportionalNumbersSelector}, 
{"tnum", kNumberSpacingType, kMonospacedNumbersSelector}, 
{"smcp", kLowerCaseType, kLowerCaseSmallCapsSelector},
{"onum", kNumberCaseType, kLowerCaseNumbersSelector},

and then, accept stuff like the CSS font-feature-settings, with something like

sketchybar --set item label.font.features="tnum,ss07,ss13"