wgl: Be lenient when sharing contexts.

My first reading of MS docs was wrong. It says:

  All rendering contexts of a shared display list must use an identical
  pixel format. Otherwise the results depend on the implementation of
  OpenGL used.

That is, it is OK to share contexts with different pixel formats.

Adobe Premiere Pro tries to do that: share lists between a rgbx8 and a
rgba8 pixel format.

src/gallium/state_trackers/wgl/stw_context.c

@@ -113,8 +113,7 @@ DrvShareLists(
    ctx1 = stw_lookup_context_locked( dhglrc1 );
    ctx2 = stw_lookup_context_locked( dhglrc2 );
 
-   if (ctx1 && ctx2 &&
-       ctx1->iPixelFormat == ctx2->iPixelFormat) { 
+   if (ctx1 && ctx2) {
       ret = _mesa_share_state(ctx2->st->ctx, ctx1->st->ctx);
    }