videocommon: UpdateDrawRectangle cleanup

These cleanups are to better understand what is going on and should contain
no functional changes.

By moving the force checking to the top I could remove most of the same
checks taking place later and let the aspect variable be reused throughout.

Because the aspectratiohack width and height as default is set to 1 besides
when ASPECT_STRETCH is used, it only needs to be calculated for that
particular case.

I also fixed the spelling of iWhidth to just say iWidth.