Add a timeout and skip signalling to the D3D9 capture path so the API
thread no longer waits indefinitely when Present stops during loading.
Return the last successful JPEG frame as a fallback.
## Link to GitHub Issue or related Pull Request, if one exists
#746
## Description of change
* Fix `capture.get_jpg` blocking indefinitely when D3D9 screen capture
cannot
complete during game loading (e.g. no Present calls).
* Add a 2-second timeout to `graphics_capture_receive_jpeg()` and cancel
pending capture requests on timeout.
* Signal capture skip from D3D9 failure paths via
`graphics_capture_skip()`.
* Cache the last successful JPEG per screen and return it as a fallback
when a new capture fails.
Tested with **stock iOS Spice Companion** only. I have not tested
Android or
other Companion clients. The iOS client implementation differs from the
others; this fix is server-side only.
No config file, CLI option, or API schema changes.
## Testing
* Built locally with llvm-mingw cross-compiler (WSL).
* Docker build (`src/spice2x/build_docker.sh`): Pending
* Tested with stock iOS Spice Companion on iPad: Mirror remains
connected
during game loading instead of returning to KeyPad.
## Link to GitHub Issue, if one exists
n/a
## Description of change
Allow the server to override client's quality options for screen
mirroring.
On Windows / Android versions (flutter-based) clients, this is already
in the options of those apps. However, the iOS version lacks the quality
options.
## Testing
Tested with iOS and Windows clients.
jessemarthin