Background shadow is always fully blended; g:clap_background_shadow_blend has no effect

[comfortablynick]

Instructions: Replace the template text and remove irrelevant text (including this line)
Warning: if you don't fill this issue template and provide the reproducible steps the issue could be closed directly.

Environment (please complete the following information):

• OS: Arch Linux
• Neovim version: v0.9.0-dev-347+gdb407010f
• vim-clap version: 0.37
• Have you reproduced with a minimal vimrc: Yes
• Have you updated to the latest plugin version: Yes
• Have you upgraded to/compiled the latest Rust binary: Yes

Describe the bug
g:clap_background_shadow_blend has no effect. When g:clap_enable_background_shadow == v:true, the background is completely black.

Clap debug

            has maple: /home/nick/.local/share/nvim/site/pack/packer/opt/vim-clap/bin/maple
           maple info: version 0.1.37 (git v0.37), built for x86_64-unknown-linux-musl by rustc 1.66.0-nightly (c0983a9aa 2022-10-12)
.
         has +python3: 1
has py dynamic module: 0
            has ctags: ctags with JSON output support
        rustc version: rustc 1.65.0 (897e37553 2022-11-02)
    Current sync impl: Lua
     Current FileType: vim
Third Party Providers: []
       Global Options:
    let g:clap#autoload_dir = '/home/nick/.local/share/nvim/site/pack/packer/opt/vim-clap/autoload'
    let g:clap#provider_alias = {'gfiles': 'git_files', 'hist:': 'command_history', 'hist/': 'search_history'}
    let g:clap_background_shadow_blend = 50
    let g:clap_disable_bottom_top = 0
    let g:clap_disable_matches_indicator = v:false
    let g:clap_disable_optional_async = v:false
    let g:clap_disable_run_rooter = v:false
    let g:clap_enable_background_shadow = v:true
    let g:clap_enable_debug = v:false
    let g:clap_enable_icon = 0
    let g:clap_forerunner_status_sign = {'done': '•', 'running': '!', 'using_cache': '*'}
    let g:clap_insert_mode_only = v:false
    let g:clap_multi_selection_warning_silent = 0
    let g:clap_no_matches_msg = 'NO MATCHES FOUND'
    let g:clap_open_action = {'ctrl-v': 'vsplit', 'ctrl-x': 'split', 'ctrl-t': 'tab split'}
    let g:clap_open_preview = 'always'
    let g:clap_popup_border = 'single'
    let g:clap_preview_direction = 'AUTO'
    let g:clap_preview_size = 5
    let g:clap_providers_relaunch_code = '@@'
    let g:clap_search_box_border_style = 'nil'
    let g:clap_search_box_border_symbols = {'nil': ['', ''], 'curve': ['', ''], 'arrow': ['', '']}
  Provider Variables:
                     []

To Reproduce
Steps to reproduce the behavior:

1. Create the minimal vimrc min.vim:

set nocompatible
set runtimepath^=./vim-clap
let g:clap_enable_background_shadow = v:true
syntax on
filetype plugin indent on

2. Start (neo)vim with command: vim -u min.vim

3. Type '....'

4. See error

Expected behavior
I expect the background to be blended according to the variable g:clap_background_shadow_blend.

Screenshots

[liuchengxu]

It's likely an issue of neovim, I can reproduce it on Linux with v0.9.0-dev-289+gae6770653, but it's fine on macOS with v0.6.0-dev
+594-g36538417f.