Broken blitCopy function and derivatives about ace

[tehKaiN]

I've wrote ACE showcase test for this - see blitter showcase test: blits with small dst
It's incomplete and I don't quite remember how it works.

While working on bob system, this bug becomes apparent during copying BG from main bitmap to designated small buffer (16x16, 32x32 or so), such that data copied to small buffer becomes corrupted. In my notes I've emphasized bug occurence during descending mode blits and also, if buffer has bigger size than copied data then everything works ok - prob'ly unused additional bits at buffer get messed up instead.