Add overview mode with missing level 1

[lucianpls]

Skipping level 1 (or every other level) would save significant space without incurring much of a performance penalty when used within gdal. Implementing this feature across all supported tile formats would save a significant amount of space.

• Add a new overview mode
• Modify tile read to read 4 tiles from lower level and downsample to generate a top tile, using avg or nnb mode
• Modify CreateOverviews to build overviews

For best performance, tiles should be stored in 2x2 groups, including the base level