st/mesa: Set pipe_image_view::shader_access in PBO readpixels.

Commit 8b626a22b2 introduced a new
pipe_image_view::shader_access field, indicating the access mode
specified in the shader.  st/mesa's built-in PBO download shader
creates a write-only image buffer, so we should flag it as such.

Nobody uses this field yet (Iris will), so we don't need to backport
this fix to stable branches.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/state_tracker/st_cb_readpixels.c

@@ -199,6 +199,7 @@ try_pbo_readpixels(struct st_context *st, struct st_renderbuffer *strb,
       image.resource = addr.buffer;
       image.format = dst_format;
       image.access = PIPE_IMAGE_ACCESS_WRITE;
+      image.shader_access = PIPE_IMAGE_ACCESS_WRITE;
       image.u.buf.offset = addr.first_element * addr.bytes_per_pixel;
       image.u.buf.size = (addr.last_element - addr.first_element + 1) *
                          addr.bytes_per_pixel;