radv: fix scissor computation when using half-pixel viewport offset

'scale[i]' can be non-integer.

Original patch by Philip Rebohle.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106074


Fixes: 0f3de89a ("radv: Use the guard band.")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Niuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 893e19ef)