rendernode: Avoid rounding errors

rendernode: Avoid rounding errors

Compute the pattern matrix directly instead of transforming the cairo_t.

This ensures that when node_size / texture_size is some obscure floating
point value, we don't get rounding issues from scaling by it once we
draw the texture_size rectangle.

I have no actual failure where this comes in handy, but I had written
the code anyway, so decided to keep it.