interface: draw cursor. Transparent.

Out-of-bounds check is working, but cursor doesn't turn red. STRANGE. Also some more tweaks ahead, for example cursor state should be directly affected by operation in use (no spawnin' elems in blocks, no diggin' air and so on). But at least now I can see where my mouse is.

Signed-off-by: mkhropov <mkhropov@gmail.com>