In d3d10umd it is required to pass D3DKMT_PRESENT in
context_private of flush_frontbuffer, but wgl passes HDC in it.
To accomidate this when gdi_sw_winsys is created functions that
acquire and release hdc from context_private are passed.
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Feng Jiang <jiangfeng@kylinos.cn>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27333>
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