LibSoftGPU: Move alpha test into separate function

Author: https://github.com/sunverwerth
Commit: 941e9d9922
Pull-request: https://github.com/SerenityOS/serenity/pull/11568
Reviewed-by: https://github.com/Hendiadyoin1 ✅
Reviewed-by: https://github.com/Quaker762 ✅
Reviewed-by: https://github.com/gmta

Userland/Libraries/LibSoftGPU/Device.h

@@ -98,6 +98,7 @@ private:
     void rasterize_triangle(const Triangle& triangle);
     void setup_blend_factors();
     void shade_fragments(PixelQuad&);
+    bool test_alpha(PixelQuad&);
 
 private:
     RefPtr<Gfx::Bitmap> m_render_target;