Fix NodeDef backwards compatibility to 5.3.0 (minetest#11942)

1. Fixes crashes on older clients when [png is used as base image
2. Fixes liquid type assertion fails on debug builds

src/nodedef.cpp

@@ -207,7 +207,17 @@ void TileDef::serialize(std::ostream &os, u16 protocol_version) const
  u8 version = 6;
  writeU8(os, version);
 
- os << serializeString16(name);
+ if (protocol_version > 39) {
+ os << serializeString16(name);
+ } else {
+ // Before f018737, TextureSource::getTextureAverageColor did not handle
+ // missing textures. "[png" can be used as base texture, but is not known
+ // on older clients. Hence use "blank.png" to avoid this problem.
+ if (!name.empty() && name[0] == '[')
+ os << serializeString16("blank.png^" + name);
+ else
+ os << serializeString16(name);
+ }
  animation.serialize(os, version);
  bool has_scale = scale > 0;
  u16 flags = 0;
@@ -491,7 +501,16 @@ void ContentFeatures::serialize(std::ostream &os, u16 protocol_version) const
  writeU32(os, damage_per_second);
 
  // liquid
- writeU8(os, liquid_type);
+ LiquidType liquid_type_bc = liquid_type;
+ if (protocol_version <= 39) {
+ // Since commit 7f25823, liquid drawtypes can be used even with LIQUID_NONE
+ // solution: force liquid type accordingly to accepted values
+ if (drawtype == NDT_LIQUID)
+ liquid_type_bc = LIQUID_SOURCE;
+ else if (drawtype == NDT_FLOWINGLIQUID)
+ liquid_type_bc = LIQUID_FLOWING;
+ }
+ writeU8(os, liquid_type_bc);
  os << serializeString16(liquid_alternative_flowing);
  os << serializeString16(liquid_alternative_source);
  writeU8(os, liquid_viscosity);