radv: Rework guard band calculation.

We want the guardband_x/y to be the largerst scalars such that each
viewport scaled by that amount is still a subrange of [-32767, 32767].

The old code has a couple of issues:
1) It used scissor instead of viewport_scissor, potentially taking into
   account a viewport that is too small and therefore selecting a scale
   that is too large.
2) Merging the viewports isn't ideal, as for example viewports with
   boundaries [0,1] and [1000, 1001] would allow a guardband scale of ~30k,
   while their union [0, 1001] only allows a scale of ~32.

The new code just determines the guardband per viewport and takes the minimum.

Signed-off-by: Bas Nieuwenhuizen <[email protected]>
Acked-by: Dave Airlie <[email protected]>

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