Apply default features for empty slice, not just nil.

[grisha]

Fixes boxesandglue/boxesandglue#14

When callers pass an empty feature slice (e.g., make([]Feature, 0) or []Feature{}), the default features including ccmp are not applied. This is because the check features == nil only matches a nil slice, not an empty one.

This causes issues when using textshape through boxesandglue, which initializes features with make([]Feature, 0, ...). The result is that GSUB features like ccmp (glyph composition/decomposition) are skipped, leading to incorrect rendering of combining marks.

For example, in Church Slavonic text, the letter И (U+0418) can have two combining diacritical marks: psili (U+0486) and oxia (U+0301). The font's ccmp feature substitutes these two marks into a single precomposed ligature glyph (uni04860301) that positions them correctly side-by-side. Without ccmp, the marks are rendered separately using mkmk positioning, which in some fonts results in overlapping or incorrectly stacked marks.

Before the fix:

After the fix:

[pgundlach]

Looks good to me, I am currently working on textshape, I will include this when I merge my changes (hopefully today)

[pgundlach]

I think that the current implementation in textshape is wrong. I will commit a change, then I will check if this issue persists. => I might not merge your pull request, but come up with a different solution. I hope you don't mind.