v0.13.0: Rendering pipeline with framebuffer backend

New module: src/rendering/framebuffer.lisp (tangled from org/framebuffer.org)

- framebuffer-backend class: implements backend protocol by writing to
  2D cell array instead of emitting escape sequences
- cell struct: per-cell state (char, fg, bg, bold, italic, underline, link-url)
- make-framebuffer / framebuffer-width / framebuffer-height
- draw-text, draw-rect, draw-border, draw-link, draw-ellipsis methods
- diff-framebuffers: compares two framebuffers, returns changed cells
- flush-framebuffer: diff + output changes to real backend
- with-scissor macro: clip drawing operations to rectangle
- cursor-move: added default no-op method for all backends
- 20 new tests, all passing (372 total)

Version bumped from 0.11.0 to 0.13.0.
License field set to GPL-3.0 in ASDF.

backend/classes.lisp

@@ -30,7 +30,8 @@
 
 (defgeneric draw-ellipsis (backend x y width &key fg bg))
 
-(defgeneric cursor-move (backend x y))
+(defgeneric cursor-move (backend x y)
+  (:method ((b backend) x y) (declare (ignore x y)) (values)))
 
 (defgeneric cursor-hide (backend)
   (:method ((b backend)) (values)))