VideoCore: Saturate vertex colors before interpolating

During testing, it was discovered that hardware does not interpolate
colors output by the vertex shader as-is. Rather, it drops the sign and
saturates the value to 1.0. This is done before interpolation, such that
(e.g.) interpolating outputs 1.5 and -0.5 is equivalent to as if the
shader had output the values 1.0 and 0.5 instead, with the interpolated
value never crossing 0.0.

This change has been tested against hardware.

src/video_core/vertex_shader.cpp

@@ -610,6 +610,12 @@ OutputVertex RunShader(const InputVertex& input, int num_attributes, const Regs:
         }
     }
 
+    // The hardware takes the absolute and saturates vertex colors like this, *before* doing interpolation
+    for (int i = 0; i < 4; ++i) {
+        ret.color[i] = float24::FromFloat32(
+            std::fmin(std::fabs(ret.color[i].ToFloat32()), 1.0f));
+    }
+
     LOG_TRACE(Render_Software, "Output vertex: pos (%.2f, %.2f, %.2f, %.2f), col(%.2f, %.2f, %.2f, %.2f), tc0(%.2f, %.2f)",
         ret.pos.x.ToFloat32(), ret.pos.y.ToFloat32(), ret.pos.z.ToFloat32(), ret.pos.w.ToFloat32(),
         ret.color.x.ToFloat32(), ret.color.y.ToFloat32(), ret.color.z.ToFloat32(), ret.color.w.ToFloat32(),