checkasm: Improve looprestoration input randomization
checkasm: Improve looprestoration input randomization

SGR uses edge detection to decide which pixels to modify, but if the
input is pure random noise there isn't going to be many (if any) edges.
As a result the entire function call often ends up doing nothing,
which isn't ideal when we want test code for correctness.

Change the input randomization algorithm to generate a checkerboard
pattern with limited noise applied to the flat areas.

videolan/dav1d