BlitCopy support for wDstX < wSrcX

[approxit]

It finally happened. After around 7 years, we just got use case for blitting with wDstX < wSrcX, which currently is not implemented.

In Last-Minute-Creations/villages@77b6cd0 I've prepared bugged version of blitting (hit space to actually render bitmap). In this use case, I'm trying to slide a bitmap from offscreen left side to center. Because we don't want to blit outside of buffer, we need to draw smaller chunks of bitmap. Details shown in below picture, made with such a professional tool:

[tehKaiN]

This requires blitting in descending mode so that pixels are blitted from bottom-right corner instead of top-left one as well as it will reverse the pixel shifting direction from right to left.