nv2a: Make multiplication by 0 match HW behavior.

Fixes xemu-project#1008

The nv2a returns 0 for anything multiplied by zero, including exceptional
values such as Inf and NaN. Desktop GPUs do not enforce this, leading to
conditions where NaNs wipe out calculations and lead to erroneous behavior.

[Test](https://github.com/abaire/nxdk_vsh_tests/blob/main/src/tests/americasarmyshader.cpp)
[HW Results](https://github.com/abaire/nxdk_vsh_tests_golden_results/wiki/Results-AmericasArmyShader)

hw/xbox/nv2a/vsh.c

@@ -636,7 +636,16 @@ static const char* vsh_header =
  "#define MUL(dest, mask, src0, src1) dest.mask = _MUL(_in(src0), _in(src1)).mask\n"
  "vec4 _MUL(vec4 src0, vec4 src1)\n"
  "{\n"
- " return src0 * src1;\n"
+ // Unfortunately mix() falls victim to the same handling of exceptional
+ // (inf/NaN) handling as a multiply, so per-component comparisons are used
+ // to guarantee HW behavior (anything * 0 must == 0).
+ " vec4 zero_components = sign(src0) * sign(src1);\n"
+ " vec4 ret = src0 * src1;\n"
+ " if (zero_components.x == 0.0) { ret.x = 0.0; }\n"
+ " if (zero_components.y == 0.0) { ret.y = 0.0; }\n"
+ " if (zero_components.z == 0.0) { ret.z = 0.0; }\n"
+ " if (zero_components.w == 0.0) { ret.w = 0.0; }\n"
+ " return ret;\n"
  "}\n"
  "\n"
  "#define ADD(dest, mask, src0, src1) dest.mask = _ADD(_in(src0), _in(src1)).mask\n"
@@ -648,7 +657,7 @@ static const char* vsh_header =
  "#define MAD(dest, mask, src0, src1, src2) dest.mask = _MAD(_in(src0), _in(src1), _in(src2)).mask\n"
  "vec4 _MAD(vec4 src0, vec4 src1, vec4 src2)\n"
  "{\n"
- " return src0 * src1 + src2;\n"
+ " return _MUL(src0, src1) + src2;\n"
  "}\n"
  "\n"
  "#define DP3(dest, mask, src0, src1) dest.mask = _DP3(_in(src0), _in(src1)).mask\n"