In situations where the gdalwarp output chunk does not overlap input data
from the current file, a great deal of work is still done by the warper.
It is desirable:
1) To short circuit chunks with no input data.
2) To shrink chunks for which there is not much coverage by input data.
The SKIP_NOSOURCE warp option can be enabled to do (1), but it is not on
by default - presumably because there are some aspects of processing that
we might need to do whether there is source data or not. For instance
initializing the destination buffer to a background value. Also, possibly
producing the various output masks.
This needs some thought, and a more holistic solution than hacks like
SKIP_NOSOURCE.
