Map tiling with oblique projection fails

[PaulWessel]

This works (no tiling since one grid):

gmt grdimage @earth_relief_06m -R-3000/2600/-790/810+uk -JOc190/25/266/68/15c -B -png m6

This looks very sad (all tiles are present on disk):

gmt grdimage @earth_relief_05m -R-3000/2600/-790/810+uk -JOc190/25/266/68/15c -B -png m5

[PaulWessel]

Notes to self:This fails for -Rxmin/xmax/ymin/ymax+uunit (even though that is an implicit oblique region) but it works for -RXLL/YLL/XUR/YUR+r for the same area. The first time -R is parsed (to determine which tiles) it works fine, but the second time it is parsed (for regular -R) something goes batty and there is confusion about w/e/s/n and oblique regions. More coffee and debugging is needed.