Orient contour labels according to the "up-slope" direction

[zstadler]

The hiking and biking maps keep the height labels on contour lines upright:

Some maps, including OpenTopoMap and MAPI, help the reader to identify the "up" direction by orienting the labels according the slope:

[zstadler]

The implementation requires the following changes, in this order:

• [x] Reverse the contour lines direction
• [x] Use "text-keep-upright": false in the contour labels style definition