SWRasterizer: Implemented stencil action 1 (GL_ZERO).

Verified with hwtests.

src/video_core/pica.h

@@ -442,7 +442,7 @@ struct Regs {
 
     enum class StencilAction : u32 {
         Keep      = 0,
-
+        Zero      = 1,
         Replace   = 2,
         Increment = 3,
         Decrement = 4,

src/video_core/rasterizer.cpp

@@ -220,6 +220,9 @@ static u8 PerformStencilAction(Regs::StencilAction action, u8 old_stencil, u8 re
     case Regs::StencilAction::Keep:
         return old_stencil;
 
+    case Regs::StencilAction::Zero:
+        return 0;
+
     case Regs::StencilAction::Replace:
         return ref;