Set sampler precision in shifted distance field text shader

The precision would default to lowp and we would attempt to
call smoothstep(mediump, mediump, lowp) which was a
non-existent overload on some drivers, thus causing a compiler
failure. We can use mediump for the sampler, like in the
regular distance field shader.

Task-number: QTBUG-35122
Change-Id: Ib50325d48fe7e0d25559da97e7f53e5170f705a1
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>