VideoCommon: Add signed version of BitfieldExtract

Source/Core/Common/BitField.h

@@ -149,6 +149,7 @@ public:
 
   constexpr T Value() const { return Value(std::is_signed<T>()); }
   constexpr operator T() const { return Value(); }
+  static constexpr bool IsSigned() { return std::is_signed<T>(); }
   static constexpr std::size_t StartBit() { return position; }
   static constexpr std::size_t NumBits() { return bits; }
 
@@ -244,6 +245,7 @@ public:
   BitFieldArray& operator=(const BitFieldArray&) = delete;
 
 public:
+  constexpr bool IsSigned() const { return std::is_signed<T>(); }
   constexpr std::size_t StartBit() const { return position; }
   constexpr std::size_t NumBits() const { return bits; }
   constexpr std::size_t Size() const { return size; }

Source/Core/VideoCommon/ShaderGenCommon.cpp

@@ -120,6 +120,12 @@ void WriteBitfieldExtractHeader(ShaderCode& out, APIType api_type,
               "  uint mask = uint((1 << size) - 1);\n"
               "  return uint(val >> off) & mask;\n"
               "}}\n\n");
+    out.Write("int bitfieldExtract(int val, int off, int size) {{\n"
+              "  // This built-in function is only supported in OpenGL 4.0+ and ES 3.1+\n"
+              "  // Microsoft's HLSL compiler automatically optimises this to a bitfield extract "
+              "instruction.\n"
+              "  return ((val << (32 - size - off)) >> (32 - size));\n"
+              "}}\n\n");
   }
 }
 

Source/Core/VideoCommon/ShaderGenCommon.h

@@ -203,7 +203,8 @@ template <auto ptr_to_bitfield_member>
 std::string BitfieldExtract(std::string_view source)
 {
   using BitFieldT = Common::MemberType<ptr_to_bitfield_member>;
-  return fmt::format("bitfieldExtract({}, {}, {})", source, static_cast<u32>(BitFieldT::StartBit()),
+  return fmt::format("bitfieldExtract({}({}), {}, {})", BitFieldT::IsSigned() ? "int" : "uint",
+                     source, static_cast<u32>(BitFieldT::StartBit()),
                      static_cast<u32>(BitFieldT::NumBits()));
 }