Restore .x models to working state (#15550)

* Add "lava flan" (.x model) smoke test

* Fix double finalize in `.x` mesh loader

* Use reserve instead of resize again

The weights are added indirectly via `AnimatedMesh->addWeight`

games/devtest/mods/testentities/models/LICENSE.txt

@@ -1,3 +1,5 @@
+"Minetest Sam":
+
 Original model by MirceaKitsune (CC BY-SA 3.0).
 Various alterations and fixes by kilbith, sofar, xunto, Rogier-5, TeTpaAka, Desour,
 stujones11, An0n3m0us (CC BY-SA 3.0):
@@ -5,3 +7,9 @@ stujones11, An0n3m0us (CC BY-SA 3.0):
 
 Jordach (CC BY-SA 3.0):
   testentities_sam.png
+
+"Lava Flan":
+
+Zeg9 (CC BY-SA 3.0):
+  testentities_lava_flan.x
+  testentities_lava_flan.png

games/devtest/mods/testentities/visuals.lua

@@ -79,6 +79,20 @@ core.register_entity("testentities:sam", {
 	end,
 })
 
+core.register_entity("testentities:lava_flan", {
+	initial_properties = {
+		infotext = "Lava Flan (smoke test .x)",
+		visual = "mesh",
+		mesh = "testentities_lava_flan.x",
+		textures = {
+			"testentities_lava_flan.png"
+		},
+	},
+	on_activate = function(self)
+		self.object:set_animation({x = 0, y = 28}, 15, 0, true)
+	end,
+})
+
 -- Advanced visual tests
 
 -- An entity for testing animated and yaw-modulated sprites