This just legalizes a few of the pixelstore pack parameters in GLES2
that are already legal in desktop and GLES3. glamor takes advantage of
this in the GetImage and software-fallback paths.
Reviewed-by: Zoltán Böszörményi <zboszor@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14977>