Refactor and rename: backends, cairo, contexts

[nikochiko]

Refactor:

• Color API
  • color fn: color : ?a:float -> int -> int -> int -> unit (rgba color making function)
  • rgb fn: rgb : int -> int -> int -> unit (rgb - opaque color making function. renamed and changed signature from opaque) - Color API: rename opaque #117
  • color type is now int * int * int * float (from int * int * int).
  • stroke defaults to Color.black for all shapes, fill defaults to Color.transparent
• Cairo backend
  • Moved from context.ml / render.ml to backend_cairo.ml and mli.
  • Stroke and fill are required for draw_{anything} functions
  • Expanded the argument types for draw_{anything} functions (was running into a cycle dependency error without - maybe that was due to something else and it isn't needed now).
  • Rename background -> set_background
  • Make set_background and set_line_width take ctx as an arg : at the backend ctx level
  • When there is a default context (context.ml), this is not needed and the global default is used
• Context API
  • type context is now a sum type over CairoBackend, SVGBackend, and LazyBackend (Context API: support multiple types of contexts #119 )
  • It has a default global context: that can be changed later. All functions that do need a context now take it as an optional argument - it is possible to work with multiple contexts parallely (Context API: support multiple contexts #118 )
  • It's also possible to set the global context and work with different backends as the default
• Shapes
  • Renamed center to origin in shape.ml code
  • Remove no_fill and no_stroke (it is the same as with_stroke Color.transparent and with_fill Color.transparent)
  • Stroke and fill are non-optional on shapes
• Examples
  • Rename line to parallel_lines and create a new simpler line example

[nikochiko]

The backend structure can change further in another PR (with #98 under review).

This can be done once we figure out a good subset of functions to take from modules such that our types can work with theirs.