Don't set default marker offset to [0,0]

Maplibre sets its own offset by default so that the "center" point of
the marker is the bottom of the marker pin. Setting the offset to [0,0]
was ovwrriding this, causing the marker to be centered in the middle of
the icon which makes it slightly inaccurate and appear to drift as the map zooms.

Fixes #237

Author: dimfeld

.changeset/neat-penguins-appear.md

@@ -0,0 +1,5 @@
+---
+'svelte-maplibre': patch
+---
+
+Default marker offset is unset instead of [0, 0]

src/lib/DefaultMarker.svelte

@@ -89,7 +89,9 @@
     marker.value?.setLngLat(lngLat);
   });
   $effect(() => {
-    marker.value?.setOffset(offset ?? [0, 0]);
+    if (offset) {
+      marker.value?.setOffset(offset);
+    }
   });
   $effect(() => {
     marker.value?.setRotation(rotation);

src/lib/Marker.svelte

@@ -128,7 +128,9 @@
     marker.value?.setLngLat(lngLat);
   });
   $effect(() => {
-    marker.value?.setOffset(offset ?? [0, 0]);
+    if (offset) {
+      marker.value?.setOffset(offset);
+    }
   });
   $effect(() => {
     marker.value?.setRotation(rotation);