When the user requests a GraphicsMode that is not directly supported
by the GPU/drivers, we should relax the requested parameters until
we find a supported mode. An exception should only be thrown when
there is no usable mode.
This makes the X11 backend match the behavior of Windows. The SDL/X11
backend works a little bit differently, in that it falls back to the
a default mode directly if the requested mode is not available. There
is nothing we can do about that.
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