don't overwrite dig priority by accident

many callers of setDesignationAt simply didn't bother with the priority
parameter. change the default value of the priority param so that by
default we will keep the previous value instead of overwriting it

docs/changelog.txt

@@ -36,6 +36,7 @@ changelog.txt uses a syntax similar to RST, with a few special sequences:
 ## New Plugins
 
 ## Fixes
+- `tiletypes`: no longer resets dig priority to the default when updating other properties of a tile
 
 ## Misc Improvements
 - `blueprint`: new ``--smooth`` option for recording all smoothed floors and walls instead of just the ones that require smoothing for later carving

library/include/modules/MapCache.h

@@ -267,7 +267,7 @@ public:
     {
         return index_tile(designation,p);
     }
-    bool setDesignationAt(df::coord2d p, df::tile_designation des, int32_t priority = 4000)
+    bool setDesignationAt(df::coord2d p, df::tile_designation des, int32_t priority = 0)
     {
         if(!valid) return false;
         dirty_designations = true;
@@ -276,6 +276,12 @@ public:
         index_tile(designation,p) = des;
         if((des.bits.dig || des.bits.smooth) && block) {
             block->flags.bits.designated = true;
+            // if priority is not specified, keep the existing priority if it
+            // is set. otherwise default to 4000.
+            if (priority <= 0)
+                priority = priorityAt(p);
+            if (priority <= 0)
+                priority = 4000;
             setPriorityAt(p, priority);
         }
         return true;
@@ -554,8 +560,8 @@ class DFHACK_EXPORT MapCache
         Block * b= BlockAtTile(tilecoord);
         return b ? b->DesignationAt(tilecoord) : df::tile_designation();
     }
-    // priority is optional, only set if >= 0
+    // if priority is 0, it is kept unchanged if previously set, otherwise 4000
-    bool setDesignationAt (DFCoord tilecoord, df::tile_designation des, int32_t priority = 4000)
+    bool setDesignationAt (DFCoord tilecoord, df::tile_designation des, int32_t priority = 0)
     {
         if (Block *b = BlockAtTile(tilecoord))
         {

library/modules/MapCache.cpp

@@ -303,7 +303,7 @@ int32_t MapExtras::Block::priorityAt(df::coord2d pos)
 
 bool MapExtras::Block::setPriorityAt(df::coord2d pos, int32_t priority)
 {
-    if (!block || priority < 0)
+    if (!block || priority <= 0)
         return false;
 
     auto event = getPriorityEvent(block, true);