gallium: allow setting of the internal stream output offset

D3D10 allows setting of the internal offset of a buffer, which is
in general only incremented via actual stream output writes. By
allowing setting of the internal offset draw_auto is capable
of rendering from buffers which have not been actually streamed
out to. Our interface didn't allow. This change functionally
shouldn't make any difference to OpenGL where instead of an
append_bitmask you just get a real array where -1 means append
(like in D3D) and 0 means do not append.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/docs/source/context.rst

@@ -182,10 +182,11 @@ discussed above.
   use pipe_so_target_reference instead.
 
 * ``set_stream_output_targets`` binds stream output targets. The parameter
-  append_bitmask is a bitmask, where the i-th bit specifies whether new
-  primitives should be appended to the i-th buffer (writing starts at
-  the internal offset), or whether writing should start at the beginning
-  (the internal offset is effectively set to 0).
+  offset is an array which specifies the internal offset of the buffer. The
+  internal offset is, besides writing, used for reading the data during the
+  draw_auto stage, i.e. it specifies how much data there is in the buffer
+  for the purposes of the draw_auto stage. -1 means the buffer should
+  be appended to, and everything else sets the internal offset.
 
 NOTE: The currently-bound vertex or geometry shader must be compiled with
 the properly-filled-in structure pipe_stream_output_info describing which