Fix and clean up skeletal animation (#15722)

* Fix attachments lagging behind their parents (#14818)
* Fix animation blending (#14817)
* Bring back cool guy as another .x smoke test
* Add .x mesh loader unittest
* Do bounding box & matrix calculation at proper point in time
* Remove obsolete `SAnimatedMesh`

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

@@ -12,4 +12,10 @@ Jordach (CC BY-SA 3.0):
 
 Zeg9 (CC BY-SA 3.0):
   testentities_lava_flan.x
-  testentities_lava_flan.png
+  testentities_lava_flan.png
+
+"Cool Guy":
+
+hecks (refer to irr/LICENSE):
+  testentities_cool_guy.x
+  testentities_cool_guy.png

games/devtest/mods/testentities/visuals.lua

@@ -102,6 +102,19 @@ core.register_entity("testentities:lava_flan", {
 	end,
 })
 
+core.register_entity("testentities:cool_guy", {
+	initial_properties = {
+		visual = "mesh",
+		mesh = "testentities_cool_guy.x",
+		textures = {
+			"testentities_cool_guy.png"
+		},
+	},
+	on_activate = function(self)
+		self.object:set_animation({x = 0, y = 29}, 30, 0, true)
+	end,
+})
+
 -- Advanced visual tests
 
 -- An entity for testing animated and yaw-modulated sprites