stb_image: NEON and SSE2 SIMD detection fixes.

This fixes two things. First, the logic to disable SSE2 on
GCC unless "-msse2" was not specific enough, and ended up
disabling SIMD support on NEON targets entirely. Shuffle
the detection logic around to make that bit x86-specific.

Second, 32-bit MinGW assumes 16-byte aligned stacks, but this is
not in the Windows ABI and hence DLLs and callbacks don't
necessarily provide it. This caused a crash.

This can be fixed by providing the right command-line option,
which we have no control over. As a compromise, disable the SSE2
path on MinGW unless a specific #define explained in the comments
is set. That way, we default to safe (never-crashing) behavior
unless the user explicitly signals they know what they're doing.

stb_image.h

@@ -624,15 +624,33 @@ typedef unsigned char validate_uint32[sizeof(stbi__uint32)==4 ? 1 : -1];
 #define STBI_FREE(p)       free(p)
 #endif
 
-#if defined(__GNUC__) && !defined(__SSE2__) && !defined(STBI_NO_SIMD)
+// x86/x64 detection
+#if defined(__x86_64__) || defined(_M_X64) || defined(__i386) || defined(_M_IX86)
+#define STBI_X86_TARGET
+#endif
+
+#if defined(__GNUC__) && defined(STBI_X86_TARGET) && !defined(__SSE2__) && !defined(STBI_NO_SIMD)
 // gcc doesn't support sse2 intrinsics unless you compile with -msse2,
 // (but compiling with -msse2 allows the compiler to use SSE2 everywhere;
 // this is just broken and gcc are jerks for not fixing it properly
 // http://www.virtualdub.org/blog/pivot/entry.php?id=363 )
 #define STBI_NO_SIMD
 #endif
 
-#if !defined(STBI_NO_SIMD) && (defined(__x86_64__) || defined(_M_X64) || defined(__i386) || defined(_M_IX86))
+#if defined(__MINGW32__) && defined(STBI_X86_TARGET) && !defined(STBI_MINGW_ENABLE_SSE2) && !defined(STBI_NO_SIMD)
+// 32-bit MinGW wants ESP to be 16-byte aligned, but this is not in the
+// Windows ABI and VC++ as well as Windows DLLs don't maintain that invariant.
+// As a result, enabling SSE2 on 32-bit MinGW is dangerous when not
+// simultaneously enabling "-mstackrealign".
+//
+// See https://github.com/nothings/stb/issues/81 for more information.
+//
+// So default to no SSE2 on 32-bit MinGW. If you've read this far and added
+// -mstackrealign to your build settings, feel free to #define STBI_MINGW_ENABLE_SSE2.
+#define STBI_NO_SIMD
+#endif
+
+#if !defined(STBI_NO_SIMD) && defined(STBI_X86_TARGET)
 #define STBI_SSE2
 #include <emmintrin.h>