Fixes a crash in offscreen Metal Swing rendering (`SkiaSwingLayer`) when
running against a Skia build with Metal API validation enabled.

`MetalSwingRedrawer.mm` (`makeMetalTexture`) creates the offscreen
texture with `texture2DDescriptorWithPixelFormat:...`, which defaults
`.usage` to `MTLTextureUsageShaderRead`. The same texture is then
wrapped as a `GrBackendRenderTarget` and rendered into, so on a debug
Skia `GrMtlGpu::onWrapBackendRenderTarget` asserts:

```
check(MTLTextureUsageRenderTarget & mtlTexture.usage)
```

and the process SIGTRAPs on the first paint. On a release Skia the
assert is compiled out and Metal tolerates the missing flag, so the bug
is latent there (and CI, which builds against release Skia, never sees
it). Present since `MetalSwingRedrawer` was added (v0.144.6), unchanged
on current `master`.

The fix sets the usage bits explicitly — `RenderTarget` because Skia
renders into the texture, `ShaderRead` because it is then sampled when
blitted back to `Graphics2D`:

```objc
textureDescriptor.usage = MTLTextureUsageRenderTarget | MTLTextureUsageShaderRead;
```

Repro (stock skiko + the published Debug Skia):

```
git clone --depth 1 --branch v0.144.6 https://github.com/JetBrains/skiko.git
./gradlew :skiko:publishToMavenLocal -Pskiko.debug=true
# paint any offscreen SkiaSwingLayer on macOS -> SIGTRAP on first paint
```

Validated: `:skiko:awtTest` green; a 2400-frame `SkiaSwingLayer` soak on
the debug Skia runs leak-free (median frame 2.4 ms, RSS flat) where it
previously crashed on frame 1. No behavior change on release builds —
the change is descriptor-time only.

Release Notes - Fixes a crash in offscreen Metal Swing rendering
(`SkiaSwingLayer`) when running against a Skia build with Metal API
validation enabled.

Fixes [SKIKO-1144](https://youtrack.jetbrains.com/issue/SKIKO-1144)