VRTKernelFilteredSource does not respect nodata

[asgerpetersen]

At the moment nodata values are not recognized and are therefore included in the convolution like any other value.

I suggest the following: If the source pixel corresponding to the output pixel is nodata then set the output pixel to nodata. Otherwise do normal convolution with the exception that nodata pixels are not used when calculating input pixel sum and kernel sum.

This way results are a lot more like one (ie. I) would expect.

I have attached my version of vrtfilters.cpp which has the above mentioned modification.

[rouault]

Thanks. Applied in r15362. Test added in r15364