fix garbage polygon rendering (error in gfx3d matrix math overflows) in spectrobes: beyond the portals

desmume/src/types.h

@@ -467,13 +467,13 @@ template<typename T> inline void reconstruct(T* t) {
 
 //-------------fixed point speedup macros
 
-#ifdef _WIN32
+#ifdef _MSC_VER
 #include <intrin.h>
 #endif
 
 FORCEINLINE s64 fx32_mul(const s32 a, const s32 b)
 {
-#ifdef _WIN32
+#ifdef _MSC_VER
 	return __emul(a,b);
 #else
 	return ((s64)a)*((s64)b);
@@ -482,16 +482,26 @@ FORCEINLINE s64 fx32_mul(const s32 a, const s32 b)
 
 FORCEINLINE s32 fx32_shiftdown(const s64 a)
 {
-#ifdef _WIN32
-	return (s32)__ll_rshift(a,12);
+	s64 shifted;
+#ifdef _MSC_VER
+	shifted = __ll_rshift(a,12);
 #else
-	return (s32)(a>>12);
+	shifted = (a>>12);
 #endif
+	//either matrix math is happening at higher precision (an extra bit would suffice, I think), or the sums sent to this are saturated.
+	//tested by: spectrobes beyond the portals excavation blower
+	//(it sets very large +x,+y in the modelview matrix to push things offscreen, but the +y will overflow and become negative if we're not careful)
+	//I didnt think very hard about what would be fastest here on 32bit systems
+	//NOTE: this was intended for use in MatrixMultVec4x4_M2; it may not be appropriate for other uses of fx32_shiftdown.
+	//if this causes problems we should refactor the math routines a bit to take care of saturating in another function
+	if(shifted>(s32)0x7FFFFFFF) return 0x7FFFFFFF;
+	else if(shifted<=(s32)0x80000000) return 0x80000000;
+	else return shifted;
 }
 
 FORCEINLINE s64 fx32_shiftup(const s32 a)
 {
-#ifdef _WIN32
+#ifdef _MSC_VER
 	return __ll_lshift(a,12);
 #else
 	return ((s64)a)<<12;