Allow overlap with gfx_CopyRectangle

[ZERICO2005]

Currently, gfx_CopyRectangle does not allow for two buffers to overlap. However, it does not appear to be too difficult to allow overlapping buffers.

This basically means implementing gfx_CopyRectangle like memmove instead of memcpy. Copying forwards with LDIR when src > dst, and copying backwards with LDDR when src < dst