console: refactor code to correctly write to the buffer offsets

[DacoTaco]

this was reported to me that the console change from some time ago broke all kind of stuff.
these changes solves it once and for all.

this was tested with the following console parameters :

const u32 startXPos = 70;
const u32 startYPos = 114;
const u32 XSize = 502;
const u32 YSize = 300;

results from CON_Init (CON_Init(xfb, startXPos, startYPos, XSize, YSize, stride );):

results from CON_InitEx(CON_InitEx(rmode, startXPos, startYPos, XSize, YSize);):

and in YAWMM :