Packed display buffer draw target for embedded-graphics

Prototype state right now, but useful for displays like SH1106 and SSD1306 which use a single byte to encode 8 pixel on/off values.

TODO/ideas/wants

• Add support for fill_contiguous as well as solid fills
• Add support for active area tracking so partial updates are possible
• Add support for 0/90/180/270º rotations
• Support bit depths > 1 and <= 8 for e.g. 2bpp 3 colour epaper displays

Test cases TODO

• Display size
  • W and H ARE multiples of 8
  • W is not a multiple of 8
  • H is not a multiple of 8
  • Neither are a multiple of 8
• Blank display
  • Perfect fill where area = display size
  • Zero sized at origin
  • Zero sized at center
  • Rectangle larger than display area on all sides
  • Rectangle half off each edge
  • Rectangle half off each corner
  • Two overlapping rectangles
  • Zero sized rectangle
  • Single pixel high rectangle
  • Single pixel wide rectangle
• Display with existing content
  • Filled rectangle over background, BinaryColor::On
  • Filled rectangle over background, BinaryColor::Off
  • Zero sized rectangle