This fixes two bugs in one go:
- setting a cursor to 0 wasn't possible when a cast+mask cursor was used
- querying a sprite for cast+mask cursor returned 0 instead of the cast
ids.
Cursors can be set in two ways:
1) set cursor of sprite X to INT
2) set cursor of sprite X to [INT, INT]
A `cursor of sprite X` should return the same values that as were used
when setting the cursor.
