Implement higher resolution modes

[0xABADCAFE]

Idea for IPixelled only (i.e no character overdraw) 2x vertical resolution:

• Use double height pixel array
• In the output, use the block mode upper half filled character as a basic screen fill.
• For every even pixel scanline, set the character cell foreground colour
• For every odd pixel scanline, set the character cell background colour

Idea for IPixelled only (i.e. no character overdraw) 4x resolution:

• Use double height / double width pixel array.
• For each 2x2 block of pixels, perform a median cut colour quantisation to find the two best colours.
  • If there are 2 or fewer distinct colours we can skip this part.
• Use one of colour as the character cell background colour and the other as foreground.
• For the 2x2 pattern of background/foreground, choose the appropriate quadrant mode block character glyph.
• This mode is likely to be significantly computationally expensive so make sure it uses an asynchronous draw process.