MOUSE clip the pointer at Y=86 when providing hotspot parameter

[LeFauve]

Test Environment (required)

• Platform: MEGA65 R6
• ROM Release: 920421
• Core Commit: 03b24c6

Describe the bug
Using the MOUSE keyword in BASIC with more than 3 parameters clips the mouse movement to the top part of the screen (RMOUSE,Y:?Y displays 86 when the mouse pointer is at the lowest possible position).
Note that if you move the mouse up, it can totally disappear in the border area.

To Reproduce
Steps to reproduce the behavior:

1. Enter this command to the BASIC prompt: mouse on,1,0,0,0
2. Move the mouse down
3. See error

Expected behavior
This particular command shouldn't have any effect since (0,0) is supposed to be the default values for the hotspot parameter.
And in any case, it shouldn't clip the mouse movement.

Screenshots
N/A

Additional context

• Using other values than (0,0) for the hotspot sometimes (not always) move the mouse pointer by a few pixels, but the behaviour doesn't seem to show that the new hotspot is taken into account (for example if I move the hotspot to (2,2), I would expect that the mouse can move 2 pixels inside the left and top border, so that the hotspot pixel of the mouse would be at (24,50))
• Also the hotspot is supposed to be made from two values, but there is no error message if we only provide one (this is correctly handled for the pos parameter: if you only provide one value for the pos parameter, you get a Syntax Error)

[dansanderson]

Repro'd.