add workaround for flickering (black frames) during zoom

inc/dd.h

@@ -168,6 +168,7 @@ typedef struct CNCDDRAW
 
     struct
     {
+        LONG frame_skip;
         BOOL enabled;
     } zoom;
 } CNCDDRAW;

src/ddsurface.c

@@ -418,6 +418,9 @@ HRESULT dds_Blt(
 
     if ((This->caps & DDSCAPS_PRIMARYSURFACE) && g_ddraw.ref && g_ddraw.render.run)
     {
+        if (!(dwFlags & DDBLT_COLORFILL))
+            InterlockedExchange(&g_ddraw.zoom.frame_skip, 0);
+
         InterlockedExchange(&g_ddraw.render.surface_updated, TRUE);
 
         if (!(This->flags & DDSD_BACKBUFFERCOUNT) || This->last_flip_tick + FLIP_REDRAW_TIMEOUT < timeGetTime())

src/dllmain.c

@@ -152,6 +152,7 @@ void DDEnableZoom()
     TRACE("%s [%p]\n", __FUNCTION__, _ReturnAddress());
 
     g_ddraw.zoom.enabled = TRUE;
+    InterlockedExchange(&g_ddraw.zoom.frame_skip, 20);
 }
 
 BOOL DDIsWindowed()

src/render_d3d9.c

@@ -552,6 +552,9 @@ DWORD WINAPI d3d9_render_main(void)
         (g_config.minfps < 0 || WaitForSingleObject(g_ddraw.render.sem, timeout) != WAIT_FAILED) &&
         g_ddraw.render.run)
     {
+        if (InterlockedDecrement(&g_ddraw.zoom.frame_skip) > 0)
+            continue;
+
 #if _DEBUG
         dbg_draw_frame_info_start();
 #endif

src/render_ogl.c

@@ -887,6 +887,9 @@ static void ogl_render()
         (g_config.minfps < 0 || WaitForSingleObject(g_ddraw.render.sem, timeout) != WAIT_FAILED) &&
         g_ddraw.render.run)
     {
+        if (InterlockedDecrement(&g_ddraw.zoom.frame_skip) > 0)
+            continue;
+
 #if _DEBUG
         dbg_draw_frame_info_start();
 #endif