Add crash command capture to preserve crashing API call in trace

[vrdaveb]

When a Vulkan driver crashes during an API call, the capture layer currently loses the offending command because parameter encoding happens post-call. This adds a new opt-in capture mode that records which API call is in-flight at the time of a crash.

When enabled via GFXRECON_CAPTURE_CRASH_COMMAND=true:

• Before each driver dispatch, a pre-encoded function call block (containing the call ID and thread ID) is saved in thread-local storage
• Crash signal handlers (SIGSEGV/SIGABRT/SIGBUS/SIGFPE/SIGILL on POSIX, SEH on Windows) write any in-flight call blocks to the capture file and flush
• Force-flush is activated so all completed calls are on disk
• After the driver call returns, the in-flight marker is cleared

The crashing function appears as the last block in the capture file, identifiable by tools like gfxrecon-convert. All preceding calls are preserved with full parameters.

Performance impact when disabled: zero (early return on bool check). When enabled: one atomic store + small memcpy per API call, plus force-flush overhead.

Fixes #699

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