st/wgl: fix double-present on swapbuffers bug

The stw_st_framebuffer_present_locked() function was getting called
twice per SwapBuffers.  First, when st_context_iface::flush() was
called from DrvSwapBuffers() because the ST_FLUSH_FRONT flag was
given.  Second, by stw_st_swap_framebuffer_locked() which does the
actual SwapBuffers.

Two code changes:
1. Pass ST_FLUSH_END_OF_FRAME, instead of ST_FLUSH_FRONT.
2. Move the implementation of stw_flush_current_locked() into
DrvSwapBuffers() since it's not called anywhere else.

Not much change in perf for benchmarks like Lightsmark, but some simple
Mesa demos are measurably faster.

Reviewed-by: José Fonseca <jfonseca@vmware.com>

src/gallium/state_trackers/wgl/stw_context.c

@@ -473,18 +473,6 @@ fail:
    return ret;
 }
 
-/**
- * Flush the current context if it is bound to the framebuffer.
- */
-void
-stw_flush_current_locked( struct stw_framebuffer *fb )
-{
-   struct stw_context *ctx = stw_current_context();
-
-   if (ctx && ctx->current_framebuffer == fb) {
-      ctx->st->flush(ctx->st, ST_FLUSH_FRONT, NULL);
-   }
-}
 
 /**
  * Notify the current context that the framebuffer has become invalid.

src/gallium/state_trackers/wgl/stw_context.h

@@ -60,7 +60,6 @@ HDC stw_get_current_dc( void );
 
 BOOL stw_make_current( HDC hdc, DHGLRC dhglrc );
 
-void stw_flush_current_locked( struct stw_framebuffer *fb );
 void stw_notify_current_locked( struct stw_framebuffer *fb );
 
 #endif /* STW_CONTEXT_H */

src/gallium/state_trackers/wgl/stw_framebuffer.c

@@ -605,15 +605,20 @@ DrvSwapBuffers(HDC hdc)
       return TRUE;
    }
 
-   /* Display the HUD */
    ctx = stw_current_context();
-   if (ctx && ctx->hud) {
-      struct pipe_resource *back =
-         stw_get_framebuffer_resource(fb->stfb, ST_ATTACHMENT_BACK_LEFT);
-      hud_draw(ctx->hud, back);
-   }
+   if (ctx) {
+      if (ctx->hud) {
+         /* Display the HUD */
+         struct pipe_resource *back =
+            stw_get_framebuffer_resource(fb->stfb, ST_ATTACHMENT_BACK_LEFT);
+         hud_draw(ctx->hud, back);
+      }
 
-   stw_flush_current_locked(fb);
+      if (ctx->current_framebuffer == fb) {
+         /* flush current context */
+         ctx->st->flush(ctx->st, ST_FLUSH_END_OF_FRAME, NULL);
+      }
+   }
 
    return stw_st_swap_framebuffer_locked(hdc, fb->stfb);
 }