Decreased map zoom level treshold to 80%

There are evidently more maps that have bigger diferences in bounding box
latitudes which lead to differnt resolutions for the same zoom level in
Mercator projection than maps that would have zoom level differences < 20%.

src/map/atlas.cpp

@@ -7,7 +7,7 @@
 #include "atlas.h"
 
 
-#define ZOOM_THRESHOLD 0.9
+#define ZOOM_THRESHOLD 0.8
 
 #define TL(m) ((m)->xy2pp((m)->bounds().topLeft()))
 #define BR(m) ((m)->xy2pp((m)->bounds().bottomRight()))