Add font variations API (#7859)

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This was mostly from last month, but I never got around to submitting
it.

This PR adds font variation coordinates to the `TextFormat` struct, and
uses them when rendering text. The coordinates are stored in a
`SmallVec`; I've chosen to store up to 2 inline, which makes it take up
24 bytes (the minimum possible for a `SmallVec`). The variation axis
tags are stored as the `font_types::Tag` type, which I've chosen to
re-export from `epaint::text`.

The variation coordinates are resolved to a `skrifa::Location` during
font rendering/scaling, and are cached in the same way as all the other
scaled metrics. I've renamed the `ScaledMetrics` struct to
`StyledMetrics`, since it now also contains the resolved variation
coordinates. I haven't benchmarked the performance of text layout with
variation coordinates, but the existing text layout performance is
unchanged.

I've replaced the API for manually overriding a font's weight
(https://github.com/emilk/egui/pull/7790) with an API for manually
overriding any variation coordinates via `FontTweak`. This should
support the same use case as #7790 while being substantially more
flexible.

I have *not* yet added any higher-level API for mapping style attributes
(weight, width, slant, etc) to variation coordinates or to different
font faces within a single family. That's a pretty huge can of worms,
and it'd involve rethinking the split between `FontId` and `TextFormat`
(and whether `FontId` is so big that we should provide a way to reuse
it). This API is intentionally pretty low-level for now.

Likewise, I've intentionally not used variation coordinates when
computing a font's row height. I can't think of any fonts that change
their vertical metrics depending on variation axes, so this should be
fine for now.

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

Cargo.lock

@@ -1567,6 +1567,7 @@ dependencies = [
  "ecolor",
  "emath",
  "epaint_default_fonts",
+ "font-types",
  "log",
  "mimalloc",
  "nohash-hasher",
@@ -1577,6 +1578,7 @@ dependencies = [
  "serde",
  "similar-asserts",
  "skrifa",
+ "smallvec",
  "vello_cpu",
 ]
 
@@ -1741,6 +1743,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b1e4d2d0cf79d38430cc9dc9aadec84774bff2e1ba30ae2bf6c16cfce9385a23"
 dependencies = [
  "bytemuck",
+ "serde",
 ]
 
 [[package]]
@@ -4136,6 +4139,9 @@ name = "smallvec"
 version = "1.15.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "67b1b7a3b5fe4f1376887184045fcf45c69e92af734b7aaddc05fb777b6fbd03"
+dependencies = [
+ "serde",
+]
 
 [[package]]
 name = "smithay-client-toolkit"