RTS: Fix artifacts after scaling due to improper rounding.
RTS: Use SSE2 conversion for rounding.
Rasterizer: Remove VirtualDub dependency to detect SSE2.
MainFrm: Don't overwrite last window position when window is not visible.

Fixes #4561
RTS: Optimize alpha mask creation. Over 5 times faster than before for
inversed subtitles.
Use c++11 random number generators.

In our case it doesn't matter that much, but why not?