#136 Lost surface fix for DirectDraw display path (Win32)

[Avery Lee]

When using DirectDraw on Win32, a display mode change
can cause all video memory surfaces to be discarded.
The update code in sdlmain.cpp restores the offscreen
blit surface, but doesn't restore the primary surface.
In some cases, this can cause DOSBox to stop updating
its window when running in windowed mode.

The attached patch adds the necessary line to restore
the primary surface.

Encountered on Windows XP by switching monitors or
switching a DOS prompt to full-screen mode. A somewhat
annoying workaround is to hit Ctrl+F1 and then dismiss
the keyboard mapper.

[Avery Lee]

Patch to sdlmain.cpp to restore primary surface (Win32)

[Qbix]

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Added it to the cvs.
Thank you.