Use PNG instead of JPEG for the GEBCO layer.

Potential fix for #49.
Fix ambiguous comments.
Merge remote-tracking branch 'ehrenfeu/bug/49' into tile_transparency
Quick hack to disable the "gebco:deepshade" layer.

This seems to be the one causing the milky appearance of the land regions when
switching on the "Marine Profile" layer as the PNG files served by the GWC are
white instead of transparent for these areas.
Updating: merge the new ref, not the old one :-/