Extends the short-cut of not fetching destination to also apply to
the RGB64 painting. This saves reading and converting destination pixels
when they will be fully replaced with source pixels.

Since ARGB32 was switched to using the RGB64 drawhelpers, and ARGB32 is
particularly expensive to read, this change is important to avoid
performance regression.

Change-Id: If3a2439140d6364e8429783cfa786bd000cfab45
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>