screen-share: define variable type before using as function argument

This defines void *data as pixman_region32_t * before using it in a function call.

As addressed in !321 (comment 345622), whenever possible void pointers shouldn't be used in function calls directly.

They are mainly used by generic API's so each user can define a data type as needed. But in our case, not defining the type is dangerous, since void pointers implicitly cast from/to any pointer without giving warnings.