gpu: activate alpha blending on the Direct3D backend.

src/video.cpp

@@ -131,6 +131,10 @@ void Video::Setup(ivec2 size)
     }
 
     g_d3ddevice = VideoData::d3d_dev;
+
+    g_d3ddevice->SetRenderState(D3DRS_ALPHABLENDENABLE, 1);
+    g_d3ddevice->SetRenderState(D3DRS_SRCBLEND, D3DBLEND_SRCALPHA);
+    g_d3ddevice->SetRenderState(D3DRS_DESTBLEND, D3DBLEND_INVSRCALPHA);
 #else
 #   if defined USE_GLEW && !defined __APPLE__
     /* Initialise GLEW if necessary */