gallium: when tracing is enabled for threaded drivers, trace the driver thread

generally speaking, if I'm tracing an app, I want to see what's happening to
my driver, not what's happening to tc, as tc does rewriting of command streams
which can affect the operation of the driver

use GALLIUM_TRACE_TC for previous behavior

Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10362>

docs/gallium/debugging.rst

@@ -32,6 +32,12 @@ specified file. Paths may be relative or absolute; relative paths are relative
 to the working directory.  For example, setting it to "trace.xml" will cause
 the trace to be written to a file of the same name in the working directory.
 
+.. envvar:: GALLIUM_TRACE_TC <bool> (false)
+
+If enabled while :ref:`trace` is active, this variable specifies that the threaded context
+should be traced for drivers which implement it. By default, the driver thread is traced,
+which will include any reordering of the command stream from threaded context.
+
 .. envvar:: GALLIUM_TRACE_TRIGGER <string> ("")
 
 If set while :ref:`trace` is active, this variable specifies a filename to monitor.