Implement support for framebuffers that don't start on a page boundary.

Implement support for framebuffers that don't start on a page boundary.
This happens on the new 14" and 16" Macbook Pro where we deliberately use
a framebuffer that skips the first few lines to avoid "the notch".
The offset of the first pixel is added to struct wsdisplay_fbinfo.  The
stride is added as well, mirroring the value returned by the
WSDISPLAYIO_LINEBYTES ioctl, such that we can retire that one in the
future.  A compat ioctl is implemented to help the transition.  The compat
code will be removed after OpenBSD 7.3 has been released.

ok miod@