In a previous PR, `mozjpeg` was set to true which might be the reason
output jpeg spikes cpu.
- https://github.com/vercel/next.js/pull/65846
I made a benchmark and found that disabling MozJPEG saved 72% CPU, with
average outputs 31% larger.
- https://github.com/lovell/sharp/issues/4603
So this PR adds a new experimental flag to enable (or rather disable)
mozjpeg and try with real workloads. This is useful becaues in many
cases, cpu is more costly than bandwidth since most CDNs provide [near
unlimited bandwidth](https://vercel.com/blog/introducing-flat-rate-cdn).
This PR removes some unused code from the image optimizer.
- Remove redundant MIME validation
- Remove unreachable fallback error
These used to be necessary until PR
https://github.com/vercel/next.js/pull/82118 removed the fallback.
Before 82118:
```js
upstreamType = detectContentType(upstreamBuffer) || imageUpstream.contentType?.toLowerCase().trim()
```
After 82118:
```js
upstreamType = detectContentType(upstreamBuffer)
```
Now there is never a case where upstreamType is invalid since its
already been validated, there is no more fallback.
This PR is strictly a refactor, no logic should change.
The idea is that we eventually want to run the image optimizer transform
step in a child process. This PR refactors the code to make a new
`transform.ts` that is lightweight and should reduce some of the
overhead of loading a large module graph when spawning a new process.
Steven