flag in a single call
This is much more efficient then making two calls, since the
QgsRasterBlock::isNoData() check internally calls QgsRasterBlock::value().
So by requiring API users to make the two separate calls individually,
we double the time this process takes...
In my simple test case this made map rendering of RGB satellite image tiles
go down by ~50% from ~40ms per tile to ~20ms per tile (in debug version, ahem)
