Premultiply background colors for X11

[bi4k8]

Checklist

• I have described the changes
• I have linked to any relevant GitHub issues, if applicable
• [n/a] Documentation in doc/ has been updated
• All new code is licensed under GPLv3

Description

Fixes #1416. Tested with the following config on both Wayland and X11 builds:

conky.config = {
	alignment='bottom_middle',
	background=false,
	border_width=1,
	cpu_avg_samples=2,
	draw_borders=true,
	draw_shades=true,
	double_buffer=true,
	out_to_x=true,
	out_to_wayland=true,
	own_window=true,
	own_window_transparent=false,
	own_window_colour='ffffff',
	own_window_class='Conky',
	own_window_type='desktop',
	own_window_argb_visual=true,
	own_window_argb_value = 182,
	update_interval=1.0,
	use_xft=true,
	font="Sans:size=8",
	default_color='efefef',
	default_shade_color='black',
	color1='ffffff',
}
conky.text = 
[[
test${alignr}test
${color1}${font Sans:size=8:bold}$nodename [$sysname $kernel $machine]$font
]]

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[rschell]

Resolved the transparency issue on my Debian 6.1.13 version