Benchmarking showed most time rendering in rgb64 mode was spend on
memsetting the buffers because they were not declared with a primitive
type.
This patch changes the buffers to quint64, but leaves refactoring
function arguments to a later patch in the dev branch.
Change-Id: Iacc81b0d8e9570b1975dffb85c955b0aabb096a7
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>