Misc webp performance work

Misc webp performance work

*- Dynamic pool sizing for WebP and Katex based on CPU count
- WebP decode: use RGB (3 bytes) for opaque images instead of RGBA (4 bytes)
- Add new imaging.config with new encoder method option; this is default 4 (what we had before), if have tested with value 2 for a 30% speedup on my MacBook, but that does sacrifice quality in some cases (quality/speed trade-off 0-6)
- Track hasAlpha in decode response to enable RGB optimization
- Scale image processing workers (1->2) on high-core machines
- Add WebP benchmark test

On my MacBook Pro M1 Pro, I now get these numbers compared to Hugo v0.152.0:

```
goos: darwin
goarch: arm64
pkg: github.com/gohugoio/hugo/internal/warpc
cpu: Apple M1 Pro
        │ cmp152.bench │         fix-webpbench.bench         │
        │    sec/op    │    sec/op     vs base               │
Webp-10   162.6m ± 32%   145.3m ± 21%  -10.60% (p=0.026 n=6)

        │ cmp152.bench │          fix-webpbench.bench          │
        │     B/op     │     B/op       vs base                │
Webp-10   41.69Mi ± 0%   123.79Mi ± 0%  +196.92% (p=0.002 n=6)

        │ cmp152.bench │         fix-webpbench.bench         │
        │  allocs/op   │  allocs/op    vs base               │
Webp-10    9.963k ± 3%   18.797k ± 2%  +88.67% (p=0.002 n=6)
```

For general image resize benchmarks compared to Hugo v0.152.0 (note that that's not the result of this branch):

```bash
goos: darwin
goarch: arm64
pkg: github.com/gohugoio/hugo/resources/images
cpu: Apple M1 Pro
               │ cmp152.bench │         fix-webpbench.bench         │
               │    sec/op    │    sec/op     vs base               │
ImageResize-10   84.12m ± ∞ ¹   40.70m ± ∞ ¹  -51.61% (p=0.029 n=4)
¹ need >= 6 samples for confidence interval at level 0.95

               │ cmp152.bench  │         fix-webpbench.bench         │
               │     B/op      │     B/op       vs base              │
ImageResize-10   32.37Mi ± ∞ ¹   29.25Mi ± ∞ ¹  -9.64% (p=0.029 n=4)
¹ need >= 6 samples for confidence interval at level 0.95

               │ cmp152.bench │        fix-webpbench.bench         │
               │  allocs/op   │  allocs/op    vs base              │
ImageResize-10   11.34k ± ∞ ¹   12.03k ± ∞ ¹  +6.07% (p=0.029 n=4)
¹ need >= 6 samples for confidence interval at level 0.95
```

Closes #14370