This prevents the Python GIL from being released before calling the method
(which is the default behaviour). For very cheap to call c++ methods the
cost of releasing the GIL can outweigh the cost of the c++ call, which means
it's more efficient to retain the hold on the GIL.
Ideally we'd do this everywhere, and switch to an explicit ReleaseGIL
annotation on functions which are slow or risky (raise exceptions, or
do something which can cause a GIL deadlock). But those are very tricky
to identify, so instead just explicitly hold the gil on cheap methods
which are likely to be called many times and could have an impact on
script performance.
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)
