Add support for mod script compatibility layers

This uses a new <Compatibility script="..." /> tag in a mod's XML file.
A compatbility layer is a script that runs before mod-init.lua is loaded,
and before any mod scripts are loaded when resuming a saved game.

This is a better solution than shipping a fragile wrapper
with every mod, that tends to break and then needs to be updated.
Now this wrapper is centralized, easy to use, and easy to update.

Closes #31.

files/scripts/compat/internal/oldfunctions.lua

@@ -0,0 +1,99 @@
+-- Registers old functions that have been deprecated in 1.1.1 already,
+-- but some mods may still use them.
+-- Most that had no function are dummied out, some existed but threw errors,
+-- and others were registered under different names.
+
+local WARN_FUNCTIONS =
+{
+    getAngleBetween = true,
+    getAngleBetweenEntities = true,
+    getNearestNode = true,
+    getNodeFromEntity = true,
+    healEntity = true,
+    killEntity = true,
+    entity_warpToPathStart = true,
+    sendEntityMessage = true,
+    entity_fireShot = true,
+    entity_resetTimer = true,
+}
+
+local DUMMY_FUNCTIONS =
+{
+    entity_setCollideWithAvatar = true,
+    entity_setTouchDamage = true,
+    bone_setTouchDamage = true,
+    entity_setClampOnSwitchDir = true,
+    entity_addGroupVel = true,
+    entity_avgVel = true,
+    entity_fireAtTarget = true,
+    entity_flipHToAvatar = true,
+    entity_getBehaviorType = true,
+    entity_isSaying = true,
+    entity_moveTowardsGroupCenter = true,
+    entity_moveTowardsGroupHeading = true,
+    entity_say = true,
+    entity_setAffectedBySpells = true,
+    entity_setBehaviorType = true,
+    entity_setNodeGroupActive = true,
+    entity_setSayPosition = true,
+    entity_setTouchPush = true,
+    entity_setPauseInConversation = true,
+    learnSpell = true,
+    reloadTextures = true,
+    setGameOver = true,
+    setGLNearest = true,
+    setNaijaModel = true,
+    shot_setBounceType = true,
+    showControls = true,
+    stopCursorGlow = true,
+    toggleTransitFishRide = true,
+    entity_stopTimer = true,
+}
+
+local REPLACED_FUNCTIONS =
+{
+    -- alternate names
+    entity_getPositionX = entity_x,
+    entity_getPositionY = entity_y,
+    entity_applyRandomForce = entity_addRandomVel,
+    getNodeByName = getNode,
+    getEntityByName = getEntity,
+    entity_flipTo = entity_fhTo,
+    bone_getidx = bone_getIndex,
+}
+
+----------------------------------------------------
+---- Functors to generate replacement function -----
+----------------------------------------------------
+
+local function mkwarn(name, param)
+    local err = "Dummy function: " .. name .. "() - no longer present in the current API, fix the script!"
+    return function() errorLog(err) end
+end
+
+local function dummy(name, param)
+end
+local function mkdummy(name, param)
+    return dummy
+end
+local function mkalias(name, param)
+    return assert(param, name)
+end
+
+local function makestubs(tab, gen)
+    for name, param in pairs(tab) do
+        if rawget(_G, name) then
+            errorLog("WARNING: oldfunctions.lua: function " .. name .. " already exists")
+        else
+            local f = gen(name, param)
+            rawset(_G, name, f)
+        end
+    end
+end
+
+----------------
+---- Do it! ----
+----------------
+makestubs(WARN_FUNCTIONS, mkwarn)
+makestubs(DUMMY_FUNCTIONS, mkdummy)
+makestubs(REPLACED_FUNCTIONS, mkalias)