core: WritePixel: Fix 24BPP write pixels.
core: WritePixel: Fix 24BPP write pixels.

The problem is that the offset to 4 bytes was counted from address cast
to the intptr_t which is signed type. So for big enough address it
yields negative number that is unexpected value and breaks the rest of
the code.

Signed-off-by: Cyril Hrubis <b1e90efd3b808b8e0bcf2a9a55fd8b9a779bfd81@ucw.cz>

gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim,gfxprim/gfxprim