major: Switch to forward slashes(+more) to fix compilation on Linux (#1403)

Notably also adds some metadata files for NixOS 

* add support for linux clang cross compiles

* add linux clang instructions

* un-capitalize Mob.horse.*

* update the description in flake.nix

---------

Co-authored-by: Loki <lokirautio@gmail.com>

Minecraft.Client/Xbox/Font/XUI_Font.cpp

@@ -1,5 +1,5 @@
 #include "stdafx.h"
-#include "..\..\Tesselator.h"
+#include "../../Tesselator.h"
 #include "XUI_FontData.h"
 #include "XUI_Font.h"
 

Minecraft.Client/Xbox/Font/XUI_FontData.cpp

@@ -1,9 +1,9 @@
 #include "stdafx.h"
-#include "..\..\stubs.h"
-#include "..\..\Minecraft.h"
-#include "..\..\Textures.h"
+#include "../../stubs.h"
+#include "../../Minecraft.h"
+#include "../../Textures.h"
 #include "XUI_FontData.h"
-#include "..\..\..\Minecraft.World\StringHelpers.h"
+#include "../../../Minecraft.World/StringHelpers.h"
 
 
 #define USE_NEW 0

Minecraft.Client/Xbox/Font/XUI_FontData.h

@@ -2,7 +2,7 @@
 using namespace std;
 #include <xuirender.h>
 
-#include "..\..\Common\UI\UIFontData.h"
+#include "../../Common/UI/UIFontData.h"
 
 // 4J This class is partially based of the ATG font implementation
 //--------------------------------------------------------------------------------------

Minecraft.Client/Xbox/Font/XUI_FontRenderer.cpp

@@ -2,7 +2,7 @@
 #include "XUI_FontRenderer.h"
 #include "XUI_Font.h"
 #include "XUI_FontData.h"
-#include "..\..\..\Minecraft.World\StringHelpers.h"
+#include "../../../Minecraft.World/StringHelpers.h"
 
 extern IDirect3DDevice9 *g_pD3DDevice;
 extern void GetRenderAndSamplerStates(IDirect3DDevice9 *pDevice,DWORD *RenderStateA,DWORD *SamplerStateA);