Warping - doing work for nothing

[warmerdam]

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.

[vali.system@…]

First draft patch to reduce warping window