SoftRasterizer: Fix Z-depth calculations when compiling on MSVC. Fixes the overworld map in Dragon Quest IV. (Regression from commit d81a75c.)

- Apparently, MSVC has a more strict implementation of IEEE-754 single-precision floats (with 23-bit significands) than Clang and GCC, and so we going to drop 2 LSBs during the calculation so that we're multiplying z by a 22-bit significand. Coincidentally, this now matches what we're doing with the OpenGL renderer, so this tends to better code consistency.

desmume/src/rasterize.cpp

@@ -517,7 +517,10 @@ FORCEINLINE void RasterizerUnit<RENDERER>::_pixel(const POLYGON_ATTR polyAttr, c
 	// When using z-depth, be sure to test against the following test cases:
 	// - The drawing of the overworld map in Dragon Quest IV
 	// - The drawing of all units on the map in Advance Wars: Days of Ruin
-	const u32 newDepth = (gfx3d.renderState.wbuffer) ? u32floor(w * 4096.0f) : u32floor(z * 16777215.0f);
+
+	// Note that an IEEE-754 single-precision float uses a 23-bit significand. Therefore, we will multiply the
+	// Z-depth by a 22-bit significand for safety.
+	const u32 newDepth = (gfx3d.renderState.wbuffer) ? u32floor(w * 4096.0f) : u32floor(z * 4194303.0f) << 2;
 	
 	// run the depth test
 	bool depthFail = false;