use colorwritemask better

panda/src/dxgsg8/dxGraphicsStateGuardian8.I

@@ -157,7 +157,16 @@ INLINE void DXGraphicsStateGuardian::
 set_color_writemask(UINT color_writemask) {
   if (_color_writemask != color_writemask) {
     _color_writemask = color_writemask;
-    scrn.pD3DDevice->SetRenderState(D3DRS_COLORWRITEENABLE, (DWORD)color_writemask);
+    if(scrn.d3dcaps.PrimitiveMiscCaps & D3DPMISCCAPS_COLORWRITEENABLE) {
+        // only newer HW supports this rstate
+        scrn.pD3DDevice->SetRenderState(D3DRS_COLORWRITEENABLE, (DWORD)color_writemask);
+    } else {
+        assert((color_writemask==0x0)||(color_writemask==0xFFFFFFFF));
+        if(color_writemask==0x0) {
+            enable_blend(true);
+            call_dxBlendFunc(D3DBLEND_ZERO, D3DBLEND_ONE);
+        } else enable_blend(false);
+    }
   }
 }
 

panda/src/dxgsg8/dxGraphicsStateGuardian8.cxx

@@ -4381,13 +4381,10 @@ void DXGraphicsStateGuardian::
 set_blend_mode(ColorWriteAttrib::Mode color_write_mode,
                ColorBlendAttrib::Mode color_blend_mode,
                TransparencyAttrib::Mode transparency_mode) {
-  // If color_write_mode is off, we disable writing to the color using
-  // blending.  I don't know if it is possible in DX to disable color
-  // outside of a blend mode.
+  // If color_write_mode is off, we disable writing to the colorbuffer.
   if (color_write_mode == ColorWriteAttrib::M_off) {
-    enable_blend(true);
     enable_alpha_test(false);
-    call_dxBlendFunc(D3DBLEND_ZERO, D3DBLEND_ONE);
+    set_color_writemask(0x0);
     return;
   }