QSvgFeGaussianBlur: reduce stack use in apply()

The function was flagged while trying to build QtSvg with
-Wframe-larger-than=20KiB, due to a 32KiB QVLA (32*32 Prealloc of a
sizeof 32 value_type).

Fix by using std::make_unqiue().

Like QVLA, make_unique() value-initializes the array, which the
following loops require (reads precede writes). Given that this
function also allocates two QImageData on the heap (QImage doesn't
have an SBO), the additional heap allocation for images <= 32 width
and height should be neglibible.

Amends 79d591d5a19ded4423ca46d0b7351fdad7b34087 (picked to 6.11),
which widened the QVLA's value_type from uint64_t to uint64_t[4],
causing the 20KiB limit to be exceeded.

Pick-to: 6.12 6.11
Change-Id: Idc34efdf6e4697dfa7592d4a9fa9faf1d2c41282
Reviewed-by: Robert Löhning <robert.loehning@qt.io>
Reviewed-by: Hatem ElKharashy <hatem.elkharashy@qt.io>