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.

cl-tty.asd

@@ -2,8 +2,8 @@
 (asdf:defsystem :cl-tty
   :description "Reusable Common Lisp Terminal UI Framework"
   :author "Amr Gharbeia"
-  :version "0.11.0"
-  :license "TBD"
+  :version "0.13.0"
+  :license "GPL-3.0"
   :depends-on (:fiveam :sb-posix)
   :components
   ((:module "backend"
@@ -16,6 +16,9 @@
    (:module "layout"
     :components
     ((:file "layout")))
+   (:module "src/rendering"
+    :components
+    ((:file "framebuffer")))
    (:module "src/components"
     :components
     ((:file "package")
@@ -74,7 +77,10 @@
      (:file "markdown-tests" :pathname "../../tests/markdown-tests")
      (:file "dialog-tests" :pathname "../../tests/dialog-tests")
      (:file "mouse-tests" :pathname "../../tests/mouse-tests")
-     (:file "slot-tests" :pathname "../../tests/slot-tests"))))
+      (:file "slot-tests" :pathname "../../tests/slot-tests")))
+    (:module "src/rendering"
+    :components
+    ((:file "framebuffer-tests" :pathname "../../tests/framebuffer-tests"))))
   :perform (test-op (o c)
                (let ((run (find-symbol "RUN" :fiveam))
                      (explain (find-symbol "EXPLAIN!" :fiveam)))
@@ -88,7 +94,8 @@
                                 (: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 (cond (suite-name (find-symbol suite-name pkg))