swr: Bump maximum 2D texture size to 16kx16k

This may be required by some applications, even though
not all texture formats will be below 2GB limit.

This change also increases the maximum size of render target,
that was inadvertently lowered some time ago.

Reviewed-by: Krzysztof Raszkowski <krzysztof.raszkowski@intel.com>
Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6139>