vt(4): Rename the "mouse_cursor" structure to "vt_mouse_cursor"

At the same time, "w" and "h" members are now called "width" and
"height". The goal is to have a more "public" structure, because it will
soon be passed as argument to a new callback, replacing vd_bitbltchr_t.

MFC after:	1 week
vt_vga: When clearing video memory, don't read from it

The goal is to clear the video memory, in case an application drew to
it. So the content shouldn't be loaded in the latches, it can't be
trusted anyway.

This improves a bit the window switch speed.

MFC after:	1 week