feat(egfx): composite client surface commands into pixel buffers

[Greg Lamberson (glamberson)]

Summary

• GraphicsPipelineClient decoded WireToSurface1 bitmaps but forwarded SolidFill, SurfaceToSurface, the bitmap cache, and MapSurfaceToOutput to no-op handler callbacks, so it never held the pixel state those commands operate on. Any server that uses them (grd, Windows) renders wrong.
• Adds a surface compositor to ironrdp-egfx: persistent RGBA8888 per-surface buffers plus a bitmap cache, applies every surface command, maps surfaces to the output, and accumulates the changed output regions.
• Regions are committed per EndFrame and drained via a new GraphicsPipelineClient::drain_output() -> Vec<OutputUpdate>.
• Additive: existing GraphicsPipelineHandler callbacks are retained as notifications, so current consumers are unaffected.

Validation

• cargo xtask check fmt/lints/tests/typos/locks all pass.
• 8 compositor unit tests (solid fill, frame atomicity, cache round-trip, oversized-destination clipping, source-rect clamping, reset).

Notes

• All reads and writes are clipped to their buffers; source-rect copies are clamped to the source surface so a malformed rectangle cannot drive an oversized allocation.
• OutputUpdate is #[non_exhaustive], matching BitmapUpdate and Surface; coordinates use ExclusiveRectangle.
• Independent of refactor(dvc)!: replace DVC wrappers with typed accessors #1377. The session-side render that drains this is feat(session): render the EGFX graphics pipeline output #1461.
• Part of [tracking] Client-side Graphics Pipeline (EGFX): wire the existing decode core into the connector and client stack #1464. Motivated by Client can't connect to GNOME Remote Desktop: no Graphics Pipeline (EGFX) advertised — the blocker after #1371 #1446.