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.

run-all-tests.lisp

@@ -15,7 +15,8 @@
              "tests/markdown-tests.lisp"
              "tests/dialog-tests.lisp"
              "tests/mouse-tests.lisp"
-             "tests/slot-tests.lisp"))
+             "tests/slot-tests.lisp"
+             "tests/framebuffer-tests.lisp"))
   (load f))
 
 ;; Run all test suites
@@ -29,7 +30,8 @@
                  (:cl-tty-mouse-test     "MOUSE-SUITE")
                  (:cl-tty-slot-test      "SLOT-SUITE")
                  (:cl-tty-layout-test    "LAYOUT-SUITE")
-                 (:cl-tty-modern-backend-test "MODERN-BACKEND-SUITE")))
+                 (:cl-tty-modern-backend-test "MODERN-BACKEND-SUITE")
+                 (:cl-tty-framebuffer-test "FRAMEBUFFER-SUITE")))
   (let* ((pkg (find-package (first suite)))
          (suite-name (second suite))
          (s (etypecase suite-name