Cope correctly when global warp parameters not valid

To do this, we add a warp_valid flag to WarpedMotionParams. This matches the warp_valid output from "Setup shear process" (section 7.11.3.6 of the spec). The flag is true by default because the default transformation type is IDENTITY.

This flag decides if warp parameters can be used. Now, we don't error out if warped motion params in frame header have alpha / beta / gamma / delta values which hare too large; instead we set the flag based on the output of dav1d_get_shear_params.

In dav1d_recon_b_inter, the logic to decide whether to use a warp affine prediction now takes the warp_valid flag into account.

This fixes issue #178 (closed)