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/PS3/PS3Extras/C4JSpursJob.cpp

@@ -7,7 +7,7 @@
 #include <cell/spurs.h>
 #include "C4JSpursJob.h"
 #include "C4JThread_SPU.h"
-// #include "PS3\SPU_Tasks\SPUMemTools\SPUMemTools.h"
+// #include "PS3/SPU_Tasks/SPUMemTools/SPUMemTools.h"
 
 
 using namespace cell::Spurs::JobQueue;

Minecraft.Client/PS3/PS3Extras/C4JThread_SPU.cpp

@@ -3,9 +3,9 @@
 #include "stdafx.h"
 
 #include "C4JThread_SPU.h"
-#include "PS3\SPU_Tasks\ChunkUpdate\ChunkRebuildData.h"
-#include "PS3\SPU_Tasks\CompressedTile\CompressedTileStorage_SPU.h"
-#include "PS3\SPU_Tasks\LevelRenderChunks\LevelRenderChunks.h"
+#include "PS3/SPU_Tasks/ChunkUpdate/ChunkRebuildData.h"
+#include "PS3/SPU_Tasks/CompressedTile/CompressedTileStorage_SPU.h"
+#include "PS3/SPU_Tasks/LevelRenderChunks/LevelRenderChunks.h"
 
 #define SPURS_MAX_SPU					6
 #define SPURS_PPU_THREAD_PRIORITY		2

Minecraft.Client/PS3/PS3Extras/DirectX/sal.h

@@ -157,7 +157,7 @@
 #endif
 
 #if defined(_MSC_EXTENSIONS) && !defined( MIDL_PASS ) && !defined(__midl) && !defined(RC_INVOKED)
-#include "codeanalysis\sourceannotations.h"
+#include "codeanalysis/sourceannotations.h"
 #endif
 
 //============================================================================

Minecraft.Client/PS3/PS3Extras/Ps3Types.h

@@ -71,7 +71,7 @@ extern Cnullptr nullptr;
 
 
 // #include <math.h>
-// #include "DirectX\DirectXMath.h"
+// #include "DirectX/DirectXMath.h"
 // using namespace DirectX;
 
 #define S_OK 0

Minecraft.Client/PS3/PS3Extras/ShutdownManager.cpp

@@ -1,7 +1,7 @@
 #include "stdafx.h"
 #include "ShutdownManager.h"
-#include "..\..\Common\Leaderboards\LeaderboardManager.h"
-#include "..\..\MinecraftServer.h"
+#include "../../Common/Leaderboards/LeaderboardManager.h"
+#include "../../MinecraftServer.h"
 #ifdef __PS3__
 #include "C4JSpursJob.h"