Updates stb_image from 1.33 to 2.10

Updates stb_image from 1.33 to 2.10 which deprecates stb_image.c and
uses only stb_image.h

rpcs3/Emu/SysCalls/Modules/cellGifDec.cpp

@@ -4,10 +4,8 @@
 #include "Emu/IdManager.h"
 #include "Emu/SysCalls/Modules.h"
 
-extern "C"
-{
+// STB_IMAGE_IMPLEMENTATION is already defined in stb_image.cpp
 #include "stblib/stb_image.h"
-}
 
 #include "Emu/FS/VFS.h"
 #include "Emu/FS/vfsFileBase.h"

rpcs3/Emu/SysCalls/Modules/cellJpgDec.cpp

@@ -4,10 +4,8 @@
 #include "Emu/IdManager.h"
 #include "Emu/SysCalls/Modules.h"
 
-extern "C"
-{
+// STB_IMAGE_IMPLEMENTATION is already defined in stb_image.cpp
 #include "stblib/stb_image.h"
-}
 
 #include "Emu/FS/VFS.h"
 #include "Emu/FS/vfsFileBase.h"

rpcs3/stb_image.cpp

@@ -1,9 +1,4 @@
 #include "stdafx.h"
-
-extern "C"
-{
-#pragma warning(push, 0)
-#include "stblib/stb_image.h"
-#include "stblib/stb_image.c"
-#pragma warning(pop)
-}
+// Defines STB_IMAGE_IMPLEMENTATION *once* before including stb_image.h (as said in stb_image.h's comments)
+#define STB_IMAGE_IMPLEMENTATION
+#include "stblib/stb_image.h"