New terminals and terminal options

• New terminals kittygd and kittycairo provide in-window graphics for terminal emulators that support the kitty protocol. Kitty is an alternative to sixel graphics that offers full 24-bit RGB color. See kittycairo.
• New terminal block for text-mode pseudo-graphics uses Unicode block or Braille characters to offer improved resolution compared to the dumb or caca terminals.
• New terminal webp generates a single frame or an animation sequence using webp encoding. Frames are generated using pngcairo, then encoded through the WebPAnimEncoder API exported by libwebp and libwebpmux.
• Terminals that use the same window for text entry and graphical display, including dumb, sixel, kitty, and block now respond to keyboard input during a pause mouse command. While paused, they interpret keystrokes in the same way that a mousing terminal would. See pseudo-mousing. For example the left/right/up/down arrow keys change the view angle of 3D plots and perform incremental pan/zoom steps for 2D plots.