gallium: add PIPE_CAP_PACKED_STREAM_OUTPUT

Setting this cap to 0 (default is 1) should disable packing
optimization for stream output (e.g. GL transform feedback captured
variables).

Signed-off-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2433>

src/gallium/auxiliary/util/u_screen.c

@@ -414,6 +414,9 @@ u_pipe_screen_get_param_defaults(struct pipe_screen *pscreen,
       /* Enables INTEL_blackhole_render */
       return 0;
 
+   case PIPE_CAP_PACKED_STREAM_OUTPUT:
+      return 1;
+
    default:
       unreachable("bad PIPE_CAP_*");
    }

src/gallium/include/pipe/p_defines.h

@@ -915,6 +915,7 @@ enum pipe_cap
    /* Turn draw, dispatch, blit into NOOP */
    PIPE_CAP_FRONTEND_NOOP,
    PIPE_CAP_NIR_IMAGES_AS_DEREF,
+   PIPE_CAP_PACKED_STREAM_OUTPUT,
 };
 
 /**

src/mesa/state_tracker/st_extensions.c

@@ -1420,6 +1420,9 @@ void st_init_extensions(struct pipe_screen *screen,
          consts->DisableVaryingPacking = GL_TRUE;
    }
 
+   if (!screen->get_param(screen, PIPE_CAP_PACKED_STREAM_OUTPUT))
+      consts->DisableTransformFeedbackPacking = GL_TRUE;
+
    unsigned max_fb_fetch_rts = screen->get_param(screen, PIPE_CAP_FBFETCH);
    bool coherent_fb_fetch =
       screen->get_param(screen, PIPE_CAP_FBFETCH_COHERENT);