fix(transparent): winblend and popup

[nullchilly]

fix #401

With vim.o.winblend = 0 and vim.o.pumblend = 0 (default value in neovim):

With vim.o.winblend = 50 and vim.o.pumblend = 50:

[EdenEast]

This looks good. Could you also just validate that this works in vim as well. From reading the code it looks like it should but I have been surprised before.

[nullchilly]

Pushed a fix for vim, seems to be working now