ImageMosaic eager raster computation slows down overlapping image mosaic computation

[aaime]

The current code finds all input images intersecting the tile requested in GetTile and extracts Raster for that area. However, in an overlapping case, it may be that the first image fully covers the tile, or more in general, that one of the inputs is actually never used for a given tile.

These calculation should be deferred to the first actual need of a input to avoid un-necessary processing.