particleshadergen: fix overly-large stack frame size in process()

A QSSGLayerRenderData has sizeof several dozen KiB, so allocate it on
the heap instead of the stack.

Found while trying to compile qtquick3d with
-Wframe-larger-than=20KiB. In a tool, such stack use is probably ok,
but the suppression macros aren't available outside qtbase at this
point, and, for a tool, a single additional heap allocation won't
matter much, esp. considering that we evade -ftrivial-auto-var-init's
memset() this way, too, so fix rather than suppress to avoid the
dependency to recent qtbase.

Amends 90600eedc0da155232b37d3fbcf8a0a293d776d8 (6.11).

Pick-to: 6.12 6.11
Task-number: QTBUG-149348
Change-Id: I1baf2a3de270d3500078ee4ed3bda62903b35e27
Reviewed-by: Christian Strømme <christian.stromme@qt.io>