fix(image): set winblend=0 for floatwin when use unicode placeholders (#1615)

When `winblend > 1`
(8de1dc6923/src/nvim/highlight.c (L751)),
hover image is not shown (in kitty).

Reproduce: `nvim +'set winblend=10' docs/image.md +2`

I have a `vim.o.winblend = 20` in global config before floatwin created.
That's why I notice it.

Co-authored-by: Folke Lemaitre <folke.lemaitre@gmail.com>

lua/snacks/image/doc.lua

@@ -366,6 +366,7 @@ function M.hover()
     local win = Snacks.win(Snacks.win.resolve(Snacks.image.config.doc, "snacks_image", {
       show = false,
       enter = false,
+      wo = { winblend = Snacks.image.terminal.env().placeholders and 0 or nil },
     }))
     win:open_buf()
     local updated = false