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

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

Define the void pointer as pixman_region32_t instead of using it
directly.

Void pointers are used by generic API's so each user can define
a data type as needed. Not doing this is dangerous, since void
pointers implicitly cast from/to any pointer without giving
warnings. So we must define its type instead of using it directly
in a function call.

Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>