GPU: Fix incorrect sign extension of positions

Fixes missing objects in Skullmonkeys (again).

src/common/bitfield.h

@@ -119,7 +119,7 @@ struct BitField
     }
     else if constexpr (std::is_signed_v<DataType>)
     {
-      constexpr int shift = 8 * sizeof(DataType) - BitCount + 1;
+      constexpr int shift = 8 * sizeof(DataType) - BitCount;
       return (static_cast<DataType>(data >> BitIndex) << shift) >> shift;
     }
     else

src/core/gpu.h

@@ -303,12 +303,12 @@ protected:
   {
     u32 bits;
 
-    BitField<u32, s32, 0, 12> x;
-    BitField<u32, s32, 16, 12> y;
+    BitField<u32, s32, 0, 11> x;
+    BitField<u32, s32, 16, 11> y;
   };
 
   // Sprites/rectangles should be clipped to 12 bits before drawing.
-  static constexpr s32 TruncateVertexPosition(s32 x) { return SignExtendN<12, s32>(x); }
+  static constexpr s32 TruncateVertexPosition(s32 x) { return SignExtendN<11, s32>(x); }
 
   struct NativeVertex
   {

src/core/gpu_sw.cpp

@@ -236,8 +236,8 @@ void GPU_SW::DispatchRenderCommand()
         vert.b = Truncate8(color_rgb >> 16);
 
         const VertexPosition vp{FifoPop()};
-        vert.x = TruncateVertexPosition(m_drawing_offset.x + vp.x);
-        vert.y = TruncateVertexPosition(m_drawing_offset.y + vp.y);
+        vert.x = m_drawing_offset.x + vp.x;
+        vert.y = m_drawing_offset.y + vp.y;
 
         if (textured)
         {