Add static glTF support (#14557)

Co-authored-by: Lars Mueller <appgurulars@gmx.de>
Co-authored-by: jordan4ibanez <jordan4ibanez@users.noreply.github.com>
Co-authored-by: sfan5 <sfan5@live.de>
Co-authored-by: SmallJoker <SmallJoker@users.noreply.github.com>

games/devtest/mods/gltf/LICENSE.md

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+glTF test model (and corresponding texture) licenses:
+
+* Spider (`gltf_spider.gltf`, `gltf_spider.png`):
+  * By [archfan7411](https://github.com/archfan7411)
+  * Licensed under CC0, public domain "wherever public domain carries fewer rights or legal protections"
+* Frog (`gltf_frog.gltf`, `gltf_frog.png`):
+  * By [Susybaka1234](https://sketchfab.com/3d-models/african-clawed-frog-v2-c81152c93948480c931c280d18957358)
+  * Licensed under CC-BY 4.0
+* Snow Man (`gltf_snow_man.gltf`, `gltf_snow_man.png`):
+  * By [jordan4ibanez](https://github.com/jordan4ibanez)
+  * Licensed under CC0
+* Minimal triangle, triangle without indices (`gltf_minimal_triangle.gltf`, `gltf_triangle_without_indices.gltf`)
+  * From [the glTF sample model collection](https://github.com/KhronosGroup/glTF-Sample-Models)
+  * Licensed under CC0 / public domain

games/devtest/mods/gltf/init.lua

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+local function register_entity(name, textures, backface_culling)
+	minetest.register_entity("gltf:" .. name, {
+		initial_properties = {
+			visual = "mesh",
+			mesh = "gltf_" .. name .. ".gltf",
+			textures = textures,
+			backface_culling = backface_culling,
+		},
+	})
+end
+
+-- These do not have texture coordinates; they simple render as black surfaces.
+register_entity("minimal_triangle", {}, false)
+register_entity("triangle_with_vertex_stride", {}, false)
+register_entity("triangle_without_indices", {}, false)
+do
+	local cube_textures = {"gltf_cube.png"}
+	register_entity("blender_cube", cube_textures)
+	register_entity("blender_cube_scaled", cube_textures)
+	register_entity("blender_cube_matrix_transform", cube_textures)
+end
+register_entity("snow_man", {"gltf_snow_man.png"})
+register_entity("spider", {"gltf_spider.png"})
+-- Note: Model has an animation, but we can use it as a static test nevertheless
+-- The claws rendering incorrectly from one side is expected behavior:
+-- They use an unsupported double-sided material.
+register_entity("frog", {"gltf_frog.png"}, false)
+
+minetest.register_node("gltf:frog", {
+	description = "glTF frog, but it's a node",
+	tiles = {{name = "gltf_frog.png", backface_culling = false}},
+	drawtype = "mesh",
+	mesh = "gltf_frog.gltf",
+})
+
+minetest.register_chatcommand("show_model", {
+	params = "<model> [textures]",
+	description = "Show a model (defaults to gltf models, for example '/show_model frog').",
+	func = function(name, param)
+		local model, textures = param:match"^(.-)%s+(.+)$"
+		if not model then
+			model = "gltf_" .. param .. ".gltf"
+			textures = "gltf_" .. param .. ".png"
+		end
+		minetest.show_formspec(name, "gltf:model", table.concat{
+			"formspec_version[7]",
+			"size[10,10]",
+			"model[0,0;10,10;model;", model, ";", textures, ";0,0;true;true;0,0;0]",
+		})
+	end,
+})

games/devtest/mods/gltf/mod.conf

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+name = gltf
+description = Hosts gltf test models, both for the C++ unit tests and for in-game viewing