Fix another type of buffer overrun for segmented modes. The buffer index
was not taken modulo the window size in VGLClear().

Segmented modes also need a kernel fix to almost work. The ioctl to set
the window origin is broken.

These bugs are rarely problems since non-VESA modes only need
segmentation to support multiple pages but libvgl doesn't support
multiple pages and treats these modes as non-segmented, and VESA modes
are usually mapped linearly except on old hardware so they really are
non-segmented.