Update vd_bitbltchr_t type to handle operation mask (to not touch pixels which

Update vd_bitbltchr_t type to handle operation mask (to not touch pixels which
 is not defined in mask) and bpl (bytes per source line).
Only vt_fb driver handle handle bpl yet.
Add protector for case when blitting image can be drawn partially out of screen,
like mouse cursor.

Mouse cursor and its movements works fine, copy/paste not yet.

Sponsored by:	The FreeBSD Foundation