GPU: Truncate sprite/rectangle positions to 12 bits before rendering

Fixes disappearing objects in Skullmonkeys.

src/core/gpu.h

@@ -166,7 +166,7 @@ public:
   bool ConvertScreenCoordinatesToBeamTicksAndLines(s32 window_x, s32 window_y, u32* out_tick, u32* out_line) const;
 
 protected:
-  static TickCount GPUTicksToSystemTicks(TickCount gpu_ticks)
+  static constexpr TickCount GPUTicksToSystemTicks(TickCount gpu_ticks)
   {
     // convert to master clock, rounding up as we want to overshoot not undershoot
     return static_cast<TickCount>((static_cast<u32>(gpu_ticks) * 7u + 10u) / 11u);
@@ -254,7 +254,6 @@ protected:
     }
   };
 
-  // TODO: Use BitField to do sign extending instead
   union VertexPosition
   {
     u32 bits;
@@ -263,6 +262,9 @@ protected:
     BitField<u32, s32, 16, 12> y;
   };
 
+  // Sprites/rectangles should be clipped to 12 bits before drawing.
+  static constexpr s32 TruncateVertexPosition(s32 x) { return SignExtendN<11, s32>(x); }
+
   union VRAMPixel
   {
     u16 bits;

src/core/gpu_hw.cpp

@@ -270,8 +270,8 @@ void GPU_HW::LoadVertices()
     {
       const u32 color = rc.color_for_first_vertex;
       const VertexPosition vp{m_fifo.Pop()};
-      const s32 pos_x = m_drawing_offset.x + vp.x;
-      const s32 pos_y = m_drawing_offset.y + vp.y;
+      const s32 pos_x = TruncateVertexPosition(m_drawing_offset.x + vp.x);
+      const s32 pos_y = TruncateVertexPosition(m_drawing_offset.y + vp.y);
 
       const auto [texcoord_x, texcoord_y] = UnpackTexcoord(rc.texture_enable ? Truncate16(m_fifo.Pop()) : 0);
       u16 orig_tex_left = ZeroExtend16(texcoord_x);

src/core/gpu_sw.cpp

@@ -514,24 +514,24 @@ template<bool texture_enable, bool raw_texture_enable, bool transparency_enable>
 void GPU_SW::DrawRectangle(s32 origin_x, s32 origin_y, u32 width, u32 height, u8 r, u8 g, u8 b, u8 origin_texcoord_x,
                            u8 origin_texcoord_y)
 {
-  origin_x += m_drawing_offset.x;
-  origin_y += m_drawing_offset.y;
+  const s32 start_x = TruncateVertexPosition(m_drawing_offset.x + origin_x);
+  const s32 start_y = TruncateVertexPosition(m_drawing_offset.y + origin_y);
 
   {
-    const u32 clip_left = static_cast<u32>(std::clamp<s32>(origin_x, m_drawing_area.left, m_drawing_area.right));
+    const u32 clip_left = static_cast<u32>(std::clamp<s32>(start_x, m_drawing_area.left, m_drawing_area.right));
     const u32 clip_right =
-      static_cast<u32>(std::clamp<s32>(origin_x + static_cast<s32>(width), m_drawing_area.left, m_drawing_area.right)) +
+      static_cast<u32>(std::clamp<s32>(start_x + static_cast<s32>(width), m_drawing_area.left, m_drawing_area.right)) +
+      1u;
+    const u32 clip_top = static_cast<u32>(std::clamp<s32>(start_y, m_drawing_area.top, m_drawing_area.bottom));
+    const u32 clip_bottom =
+      static_cast<u32>(std::clamp<s32>(start_y + static_cast<s32>(height), m_drawing_area.top, m_drawing_area.bottom)) +
       1u;
-    const u32 clip_top = static_cast<u32>(std::clamp<s32>(origin_y, m_drawing_area.top, m_drawing_area.bottom));
-    const u32 clip_bottom = static_cast<u32>(std::clamp<s32>(origin_y + static_cast<s32>(height), m_drawing_area.top,
-                                                             m_drawing_area.bottom)) +
-                            1u;
     AddDrawRectangleTicks(clip_right - clip_left, clip_bottom - clip_top, texture_enable);
   }
 
   for (u32 offset_y = 0; offset_y < height; offset_y++)
   {
-    const s32 y = origin_y + static_cast<s32>(offset_y);
+    const s32 y = start_y + static_cast<s32>(offset_y);
     if (y < static_cast<s32>(m_drawing_area.top) || y > static_cast<s32>(m_drawing_area.bottom))
       continue;
 
@@ -539,7 +539,7 @@ void GPU_SW::DrawRectangle(s32 origin_x, s32 origin_y, u32 width, u32 height, u8
 
     for (u32 offset_x = 0; offset_x < width; offset_x++)
     {
-      const s32 x = origin_x + static_cast<s32>(offset_x);
+      const s32 x = start_x + static_cast<s32>(offset_x);
       if (x < static_cast<s32>(m_drawing_area.left) || x > static_cast<s32>(m_drawing_area.right))
         continue;