Fix swap function

[Dwyriel]

Saves the value of pointer a to a temp t instead of the pointer itself, then puts that back into b.

Explanation:
The swap function was saving the pointer a to a temporary pointer t, then changing the value pointed by a (and thus also t) to the value pointed by b, then putting the value pointed by the temporary value t (which became the content of b after *a = *b since both t and a were the same pointer) resulting in both a and b having the same value (of b).
This would cause the function ssd1306_draw_line to only render a single dot at (x2, y2) if x1 was greater than x2 or (x1,y2) if x1 was equal to x2.