Change default channel attribution when using get_voxel_tool()

- It now depends first on the mesher
- If no mesher is assigned, it depends on the generator
- If no generator is assigned, it depends on the stream
- If no stream is assigned, it defaults to 0
- If multiple channels are found, pick the first one instead of last
- The reason is, some streams can support all channels so they dont
  represent well what the user intends to use.

terrain/voxel_node.cpp

@@ -100,16 +100,19 @@ String VoxelNode::get_configuration_warning() const {
 }
 
 int VoxelNode::get_used_channels_mask() const {
+	Ref<VoxelMesher> mesher = get_mesher();
+	if (mesher.is_valid()) {
+		return mesher->get_used_channels_mask();
+	}
 	Ref<VoxelGenerator> generator = get_generator();
-	Ref<VoxelStream> stream = get_stream();
-	int used_channels_mask = 0;
 	if (generator.is_valid()) {
-		used_channels_mask |= generator->get_used_channels_mask();
+		return generator->get_used_channels_mask();
 	}
+	Ref<VoxelStream> stream = get_stream();
 	if (stream.is_valid()) {
-		used_channels_mask |= stream->get_used_channels_mask();
+		return stream->get_used_channels_mask();
 	}
-	return used_channels_mask;
+	return 0;
 }
 
 void VoxelNode::_bind_methods() {

terrain/voxel_terrain.cpp

@@ -1248,6 +1248,7 @@ Ref<VoxelTool> VoxelTerrain::get_voxel_tool() {
 	for (int channel = 0; channel < VoxelBuffer::MAX_CHANNELS; ++channel) {
 		if ((used_channels_mask & (1 << channel)) != 0) {
 			vt->set_channel(channel);
+			break;
 		}
 	}
 	return vt;