graphics/gpxsee: Update to 7.15

Changelog:

• Fixed broken align of zoom level 0 with tiles of a different size than 256px.
• Increased the default tile cache to 256MB
• IMG maps text layout tweaking and optimization.

PR: 241443
Submitted by: Nikolay Korotkiy <sikmir@gmail.com> (maintainer)
MFH: 2019Q4