Trival style fix to previous commit to add this detail.

The previous commit also fixed the coordinates passed to the mouse
removal renderer. The coordinates were rounded down to a character
boundary, and thus essentially unusable. The renderer had to keep
track of the previous position, or clear a larger area. The latter
is only safe in the border, which is all that needs special handling
anyway.

I think no renderer depends on the bug. They have the following
handling:

• gfb sparc64: this seems to assume non-rounded coordinates
• gfb other: does nothing (seems to be missing border handling)
• vga text: does nothing (doesn't need border handling)
• vga planar: clears extras in the border, with some bugs. The fixes will use the precise coordinates to optimize.
• vga direct: clears at the previous position with no check that it is active, and clears everything. Checking finds this bug.
• others: are there any?