avoid goroutine leak when creating bigtiff cogs (#7)

cog.go

@@ -473,6 +473,11 @@ func (cog *cog) computeImageryOffsets() error {
 			} else {
 				if dataOffset > uint64(^uint32(0)) { //^uint32(0) is max uint32
 					//rerun with bigtiff support
+
+					//first empty out the tiles channel to avoid a goroutine leak
+					for range tiles {
+						//skip
+					}
 					cog.bigtiff = true
 					return cog.computeImageryOffsets()
 				}