The previous implementation kept a hashtable of a Backtrace object per
thread. debug_backtrace_capture is supposed to store a backtrace in
the passed in debug_stack_frame array, but instead overwrote the
per-thread Backtrace object.
This new version works more like the libunwind based capture. We hash
the file and symbol names and store pointers in the debug_stack_frame
struct. This way debug_backtrace_capture doesn't overwrite previous
captures or allocate memory that needs to be freed.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6112>
Kristian H. Kristensen