Sparse storage for Geopackage caches

[brncsk]

Context

As per the Geopackage specification, tile matrices can be sparsely populated: any tile can be omitted from the database. In fact, this is exactly what the OGR driver does (see section 'Creation issues') to save disk space by avoiding storing fully transparent tiles. MapProxy should probably do the same.

Expected Behavior

MapProxy should refrain from writing fully transparent tiles into Geopackage files.

Actual Behavior

MapProxy writes each and every tile into the Geopackage file even if they are fully transparent.

Possible Fix

This possibly overlaps with the functionality of link_single_color_images.

Your Environment

• Version used: 1.11
• Environment name and version (e.g. Python 2.7.5 with mod_wsgi 4.5.9): Python 3.4.5
• Server type and version: N/A
• Operating System and version: SLES 11