Patch in thread local storage for stb_image vflip

This makes loading images in multiple threads concurrently safe.

Adds a new CMake variable LOVR_USE_THREADLOCAL, by default on

CMakeLists.txt

@@ -36,6 +36,8 @@ option(LOVR_BUILD_EXE "Build an executable" ON)
 option(LOVR_BUILD_SHARED "Build a shared library (takes precedence over LOVR_BUILD_EXE)" OFF)
 option(LOVR_BUILD_BUNDLE "On macOS, build a .app bundle instead of a raw program" OFF)
 
+option(LOVR_USE_THREADLOCAL "Allow use of thread local storage; disable to run on Windows XP as a DLL" ON)
+
 # Setup
 if(EMSCRIPTEN)
   string(CONCAT LOVR_EMSCRIPTEN_FLAGS
@@ -286,6 +288,9 @@ if(LOVR_ENABLE_THREAD)
     find_package(Threads REQUIRED)
     set(LOVR_PTHREADS Threads::Threads)
   endif()
+  if (LOVR_USE_THREADLOCAL)
+    add_definitions(-DUSE_LOVR_STBI_THREADLOCAL)
+  endif()
 endif()
 
 # LÖVR

src/lib/stb/stb_image.c

@@ -5,3 +5,7 @@
 #define STBI_ONLY_HDR
 #define STBI_ASSERT(x)
 #include "stb_image.h"
+
+#ifndef LOVR_STBI_VFLIP_PATCH
+#error "Somebody updated stb_image.h without replacing the thread-local patch"
+#endif

src/lib/stb/stb_image.h

@@ -995,7 +995,15 @@ static float   *stbi__ldr_to_hdr(stbi_uc *data, int x, int y, int comp);
 static stbi_uc *stbi__hdr_to_ldr(float   *data, int x, int y, int comp);
 #endif
 
-static int stbi__vertically_flip_on_load = 0;
+#define LOVR_STBI_VFLIP_PATCH
+#ifdef USE_LOVR_STBI_THREADLOCAL
+#include "lib/tinycthread/tinycthread.h"
+#define THREADLOCAL_IF_AVAILABLE _Thread_local
+#else
+#define THREADLOCAL_IF_AVAILABLE
+#endif
+
+static THREADLOCAL_IF_AVAILABLE int stbi__vertically_flip_on_load = 0;
 
 STBIDEF void stbi_set_flip_vertically_on_load(int flag_true_if_should_flip)
 {