Currently callers of this function are responsible for setting the draw buffer
correctly, but they don't do a very good job:
  - swapchain_init() sets the draw buffer to GL_BACK if there's a back buffer,
    even though the context's target is always the front buffer.
  - swapchain_create_context_for_thread() depends on (eventually) being called
    by FindContext().
  - create_primary_opengl_context() and
    IWineD3DSwapChainImpl_SetDestWindowOverride() don't bother setting a draw
    buffer at all.
Just marking the draw buffer dirty lets the context management sort it all
out, and is much simpler.