Split ANIMATED into three parts, filtered by game that uses them

- ANIMATED contained definitions for Doom, Heretic, and Strife, all
  crammed into a single file. This meant that animations from one game
  could erroneously make their way into maps for another game that
  provided custom textures with names that matched textures that animated
  in the other game. There are now three separate ANIMATED lumps with only
  the animations defined for the original game and no others. The one
  that gets loaded depends on the game being played.
- Added documentation for the ANIMATED file format to the comment for
  FTextureManager::InitAnimated(), since I had to figure it out from the
  code.

src/textures/animations.cpp

@@ -161,6 +161,16 @@ FAnimDef *FTextureManager::AddComplexAnim (FTextureID picnum, const TArray<FAnim
 // [RH] Rewritten to support BOOM ANIMATED lump but also make absolutely
 //		no assumptions about how the compiler packs the animdefs array.
 //
+// Since animdef_t no longer exists in ZDoom, here is some documentation
+// of the format:
+//
+//   This is an array of <n> entries, terminated by a 0xFF byte. Each entry
+//   is 23 bytes long and consists of the following fields:
+//     Byte      0: Bit 1 set for wall texture, clear for flat texture.
+//     Bytes   1-9: '\0'-terminated name of first texture.
+//     Bytes 10-18: '\0'-terminated name of last texture.
+//     Bytes 19-22: Tics per frame (stored in little endian order).
+//
 //==========================================================================
 
 CVAR(Bool, debuganimated, false, 0)