egfx client: one H.264 decoder shared by all surfaces breaks multi-surface streams #2033

GraphicsPipelineClient holds a single H.264 decoder (h264_decoder: Option<Box<dyn H264Decoder>> in crates/ironrdp-egfx/src/client.rs) and feeds it the AVC420/AVC444 payloads of every surface. A server that encodes each surface as its own H.264 stream, as MS-RDPEGFX allows (each WireToSurface1 names its surfaceId, and the surfaces can differ in size), interleaves independent streams into that one decoder. The second stream's SPS/PPS and reference frames replace the first's, and decoding fails.

Seen with a two-monitor server session (two surfaces, 1280x720 each at x=0 and x=1280, each with its own AVC420 stream): an IronRDP-based client fails within a second with OpenH264 decode failed (native error 2), while xfreerdp3 /multimon displays both monitors correctly. FreeRDP keeps an H.264 context per surface.

A decoder per surface, created when the surface is created and dropped with it, would match. That likely needs a decoder factory rather than a single decoder instance passed to GraphicsPipelineClient::new, so it is an API question as much as an implementation one.