st/mesa: Set pipe_image_view::shader_access in PBO readpixels.
Commit 8b626a22 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 <firstname.lastname@example.org>