vcl: rename variable in OutputDevice:drawOutDevDirect()

pGraphics2 is quite non-descriptive in OutputDevice::drawOutDevDirect().
It's actually the source SalGraphics instance of the source OutputDevice,
so rename it to pSrcGraphics which is rather more clear.

Change-Id: Iea8aa7410a294e012fccd9d4c8d7ff9c1270aa74