Blend the semi-transparent background color of the scrollback rows with the global background color (and fix OSC 11 behavior)

[o-sdn-o]

Changes

• Implement new transparency support logic for the built-in terminal:
  • Store an alpha channel for colors:
    • Foreground of each scrollback cell (SGR 38:2::R:G:B:A, A=255 by default)
    • Background of each scrollback cell (SGR 48:2::R:G:B:A, A=255 by default)
  • Inherit the foreground alpha for underline colors (set by SGR 58:2::R:G:B).
  • Handle the terminal window rendering by:
    • Blending the semi-transparent background color of the scrollback rows with the global background color (set by OSC 11).
    • Blending the semi-transparent foreground (and underline) color of the scrollback rows with the final background color calculated in the previous step.

An example of using color brushes that are independent of the global background:

printf "\e[m
 \e[35b\n  \e[48:2::0:0:255:128m Some Text Fragment \e[m  
 \e[35b\n  \e[48:2::0:255:0:128m Some Text Fragment \e[m  
 \e[35b\n  \e[48:2::255:0:0:128m Some Text Fragment \e[m  
 \e[35b\n  \e[48:2::0:255:255:128m Some Text Fragment \e[m  
 \e[35b\n  \e[48:2::255:255:0:128m Some Text Fragment \e[m  
 \e[35b\n  \e[48:2::255:0:255:128m Some Text Fragment \e[m  
 \e[36b\n"