BuildVRT: Warn if resulting VRT requires resampling

[dbaston]

Feature description

A common use of gdalbuildvrt is the creation of mosaics or stacks whose pixels values are equal to those of the underlying data at all locations. When this is not the case (and the underlying values are resampled) it would be nice to have a warning or optionally an error. I started adding a check for this as part of #11982 but I realized it's equally applicable to other values of --resolution, so it can be addressed separately.

Additional context

No response