`ImageElementBase.LoadImage` fetched http(s) images with a throwaway
`new HttpClient()` + `GetStreamAsync`: no timeout override (the 100s
default), no size cap, unbounded redirects, no pooling — all inside
stream startup, while ffmpeg waits on the pipe. #502 routed ordinary
channel-logo watermarks onto that path, widening a pre-existing weakness.
Introduce `IRemoteImageFetcher` / `HttpRemoteImageFetcher`, modelled on
the neighbouring `IRemoteStreamProber`:
- deadline covers headers AND body (linked CTS + `CancelAfter`, client
`Timeout = InfiniteTimeSpan`) — under `ResponseHeadersRead` the body
read falls outside `HttpClient.Timeout` (the #289 lesson)
- 10 MiB cap enforced during the copy; `Content-Length` is only a cheap
early reject, since it can be absent or a lie
- permissive content-type check (rejects an HTML error page, allows a
missing type and octet-stream)
- pooled via `IHttpClientFactory`; redirects capped at 3, not 50
A byte cap does NOT bound decoding, so `DecodeRemoteImage` additionally
reads declared dimensions + frame count from the header and rejects
before `Image.LoadAsync` allocates (50 MP / 600 frames). A 4 KB PNG
declaring 30000x30000 costs ~3.6 GB to decode and passes every wire-size
check — caught by adversarial review of the first version of this change,
which capped bytes and wrongly claimed that was decode-bomb protection.
Not cached and SSRF not mitigated — both deliberate, with the reasoning
recorded in docs/decisions.md.
fixes#511
* use skiasharp in graphics engine
* start to use richtextkit
* move out some template functions
* move files
* add base graphics element
* use default style in text element
* support partial styling in text element
* fix static images
* load fonts from text element definition
timothy
