v2: lots of improvements

This is like identity or trace, but simpler. It doesn't wrap most states.

Run with:
  GALLIUM_DDEBUG=1000 [executable]
where "executable" is the app and "1000" is in miliseconds, meaning that
the context will be considered hung if a fence fails to signal in 1000 ms.

If that happens, all shaders, context states, bound resources, draw
parameters, and driver debug information (if any) will be dumped into:
  /home/$username/dd_dumps/$processname_$pid_$index.

Note that the context is flushed after every draw/clear/copy/blit operation
and then waited for to find the exact call that hangs.

You can also do:
  GALLIUM_DDEBUG=always
to do the dumping after every draw/clear/copy/blit operation without
flushing and waiting.

Examples of driver states that can be dumped are:
- Hardware status registers saying which hw block is busy (hung).
- Disassembled shaders in a human-readable form.
- The last submitted command buffer in a human-readable form.

v2: drop pipe-loader changes, drop SConscript
    rename dd.h -> dd_pipe.h

Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>