draw: account for elem size when computing overflow

draw: account for elem size when computing overflow

We weren't taking into account the size of element
that is to be fetched, which meant that it was possible
to overflow the buffer reads if the stride was very
close to the end of the buffer, e.g. stride = 3, buffer
size = 4, and the element to be read = 4. This should
be properly detected as an overflow.

Signed-off-by: Zack Rusin <zackr@vmware.com>