Fix for sub_6E7F34 in viewport.c

I'd like @duncanspumpkin to review this as he disabled these functions and I'm not sure if I have completely removed the issues. I'm also not sure if the argument should still be called `previous_x`. The second commit is an unrelated logic fix that didn't cause any noticeable change.

BTW, a strange thing is, when I replaced the fixed line 
`RCT2_CALLPROC_X(0x6E7FF3, 0, 0, 0, previous_x, (int)viewport, (int)w, previous_y);`
with
`RCT2_CALLPROC_X(0x6E7FF3, 0, 0, 0, 0, (int)viewport, (int)w, 0);`,
It still seemed to work correctly.