DIB_DirectDrawSurface_BltFast signed/unsigned comparision bug

[Piotr Pawlow]

Hello,

I spotted this bug while trying to fix mouse pointer trails problem in 
Warlords Battlecry. All widths, heights and left/top coordinates are of DWORD 
type, so substracting them from each other always give positive numbers. Thus 
h and w are always >= 0, so DDERR_INVALIDRECT is returned only in a couple of 
boundary cases.

I just added explicit casting to int where needed. It didn't fix the mouse 
trails, but previously missing mouseover effect on icons at the main screen 
appeared, so I guess it is a change in the right direction :)

Patch attached.