pica/shader: extend UnitState for GS

Among four shader units in pica, a special unit can be configured to run both VS and GS program. GSUnitState represents this unit, which extends UnitState (which represents the other three normal units) with extra state for primitive emitting. It uses lots of raw pointers to represent internal structure in order to keep it standard layout type for JIT to access.
This unit doesn't handle triangle winding (inverting) itself; instead, it calls a WindingSetter handler. This will be explained in the following commits

src/video_core/shader/shader.cpp

@@ -82,6 +82,44 @@ void UnitState::WriteOutput(const ShaderRegs& config, AttributeBuffer& output) {
     }
 }
 
+UnitState::UnitState(GSEmitter* emitter) : emitter_ptr(emitter) {}
+
+GSEmitter::GSEmitter() {
+    handlers = new Handlers;
+}
+
+GSEmitter::~GSEmitter() {
+    delete handlers;
+}
+
+void GSEmitter::Emit(Math::Vec4<float24> (&vertex)[16]) {
+    ASSERT(vertex_id < 3);
+    std::copy(std::begin(vertex), std::end(vertex), buffer[vertex_id].begin());
+    if (prim_emit) {
+        if (winding)
+            handlers->winding_setter();
+        for (size_t i = 0; i < buffer.size(); ++i) {
+            AttributeBuffer output;
+            unsigned int output_i = 0;
+            for (unsigned int reg : Common::BitSet<u32>(output_mask)) {
+                output.attr[output_i++] = buffer[i][reg];
+            }
+            handlers->vertex_handler(output);
+        }
+    }
+}
+
+GSUnitState::GSUnitState() : UnitState(&emitter) {}
+
+void GSUnitState::SetVertexHandler(VertexHandler vertex_handler, WindingSetter winding_setter) {
+    emitter.handlers->vertex_handler = std::move(vertex_handler);
+    emitter.handlers->winding_setter = std::move(winding_setter);
+}
+
+void GSUnitState::ConfigOutput(const ShaderRegs& config) {
+    emitter.output_mask = config.output_mask;
+}
+
 MICROPROFILE_DEFINE(GPU_Shader, "GPU", "Shader", MP_RGB(50, 50, 240));
 
 #ifdef ARCHITECTURE_x86_64