CrossSIMD: fill in the scalar fallback, fix two LSX bugs

LoadConvertU8, StoreConvertToU8 and LoadTranspose existed in the SSE2,
NEON and LSX implementations but not in the scalar one, so anything using
them wouldn't build on a target without SIMD.

The two LSX bugs were found by the new CrossSIMD unit test, run under
qemu-loongarch64:

- Vec4F32::operator[] had a switch with no breaks, so every index fell
  through to the default and returned lane 3.
- StoreConvertToU8 narrowed with the logical (unsigned) saturating shifts,
  which turn a negative value into a huge unsigned one and saturate it to
  255. It should clamp to 0, as the packs/packus pair in the SSE version
  does. Narrow signed->signed and then signed->unsigned instead.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

Common/Math/CrossSIMD.h

@@ -1188,8 +1188,11 @@ struct Vec4F32 {
 	}
 	void StoreConvertToU8(uint8_t *dest) {
 		__m128i ivalue32 = __lsx_vftintrz_w_s(v);
-		__m128i ivalue16 = __lsx_vssrlrni_hu_w(ivalue32, ivalue32, 0);
-		__m128i ivalue8 = __lsx_vssrlrni_bu_h(ivalue16, ivalue16, 0);
+		// Narrow signed->signed first, then signed->unsigned, matching the packs/packus pair the
+		// SSE version uses. The logical (unsigned) narrowing shifts turn a negative value into a
+		// huge unsigned one, which saturates to 255 instead of clamping to 0.
+		__m128i ivalue16 = __lsx_vssrani_h_w(ivalue32, ivalue32, 0);
+		__m128i ivalue8 = __lsx_vssrani_bu_h(ivalue16, ivalue16, 0);
 		uint32_t value = __lsx_vpickve2gr_wu(ivalue8, 0);
 		memcpy(dest, &value, sizeof(uint32_t));
 	}
@@ -1199,10 +1202,10 @@ struct Vec4F32 {
 		// index is a compile-time constant parameter to the intrinsic.
 		int ival;
 		switch (index) {
-		case 0: ival = __lsx_vpickve2gr_w((__m128i)v, 0);
-		case 1: ival = __lsx_vpickve2gr_w((__m128i)v, 1);
-		case 2: ival = __lsx_vpickve2gr_w((__m128i)v, 2);
-		default: ival = __lsx_vpickve2gr_w((__m128i)v, 3);
+		case 0: ival = __lsx_vpickve2gr_w((__m128i)v, 0); break;
+		case 1: ival = __lsx_vpickve2gr_w((__m128i)v, 1); break;
+		case 2: ival = __lsx_vpickve2gr_w((__m128i)v, 2); break;
+		default: ival = __lsx_vpickve2gr_w((__m128i)v, 3); break;
 		}
 		float fval;
 		memcpy(&fval, &ival, sizeof(float));
@@ -1672,6 +1675,25 @@ struct Vec4F32 {
 		return temp;
 	}
 
+	static Vec4F32 LoadConvertU8(const uint8_t *src) {  // Note: will load 8 bytes, not 4. Only the first 4 bytes will be used.
+		Vec4F32 temp;
+		for (int i = 0; i < 4; i++) {
+			temp.v[i] = (float)src[i];
+		}
+		return temp;
+	}
+
+	// Truncates towards zero and saturates to 0-255, like the packing instructions the other
+	// implementations use.
+	void StoreConvertToU8(uint8_t *dest) {
+		for (int i = 0; i < 4; i++) {
+			int value = (int)v[i];
+			if (value < 0) value = 0;
+			if (value > 255) value = 255;
+			dest[i] = (uint8_t)value;
+		}
+	}
+
 	static Vec4F32 LoadF24x3_One(const uint32_t *src) {
 		constexpr uint32_t kOneF32Bits = 0x3F800000;
 		uint32_t shifted[4] = { src[0] << 8, src[1] << 8, src[2] << 8, kOneF32Bits };
@@ -1866,6 +1888,15 @@ struct Vec4F32 {
 		return Vec4F32{{ v[3], v[3], v[3], v[3] }};
 	}
 
+	// Loads four rows of four floats and transposes them into columns.
+	static void LoadTranspose(const float *src, Vec4F32 &col0, Vec4F32 &col1, Vec4F32 &col2, Vec4F32 &col3) {
+		col0 = Vec4F32::Load(src);
+		col1 = Vec4F32::Load(src + 4);
+		col2 = Vec4F32::Load(src + 8);
+		col3 = Vec4F32::Load(src + 12);
+		Transpose(col0, col1, col2, col3);
+	}
+
 	// In-place transpose.
 	static void Transpose(Vec4F32 &col0, Vec4F32 &col1, Vec4F32 &col2, Vec4F32 &col3) {
 		float m[16];