first attempt at threading the 3D renderer

src/GPU2D.cpp

@@ -380,6 +380,11 @@ void GPU2D::DrawScanline(u32 line)
 {
     u32* dst = &Framebuffer[256*line];
 
+    // request each 3D scanline in advance
+    // this is required for the threaded mode of the software renderer
+    if (Num == 0)
+        GPU3D::RequestLine(line);
+
     line = GPU::VCount;
 
     // scanlines that end up outside of the GPU drawing range