Port font editor to SDL2

Replace X11 with SDL2 to improve the portability and cross-platform support

Migrated window creation from X11's `XCreateWindow` to `SDL_CreateWindow`.

Replaced X11 event handling with SDL2’s event loop (`SDL_PollEvent`)
to capture input events such as keyboard and mouse interactions.

Updated rendering to use `SDL_Renderer` and `SDL_Surface`,
replacing X11's rendering functions.

* Modified some key event logic:
1. SDLK_ESCAPE: ESC now exits the program.
2. SDL_QUIT: Clicking the "X" on the window exits the program.

* Unchanged key event logic:
1. SDLK_q: Switches to the next font.

* Rename delete function:
1. Rename delete() to delete_char() to avoid `clang-format` misinterpreting
`delete()` as the C++ the keyword, which cauese an extra space to be
added when running `clang-format`, turning `delete()` to `delete ()`.

* Add operation instructions in the READMD.

tools/font-edit/Makefile

@@ -1,7 +1,7 @@
 TARGET = twin-fedit
 
-CFLAGS = $(shell pkg-config --cflags cairo x11) -g -Wall
-LIBS = $(shell pkg-config --libs cairo x11)
+CFLAGS = $(shell pkg-config --cflags cairo) $(shell sdl2-config --cflags) -g -Wall
+LIBS = $(shell pkg-config --libs cairo) $(shell sdl2-config --libs)
 
 OBJS = \
 	twin-fedit.o \

tools/font-edit/README.md

@@ -5,7 +5,7 @@ screens.
 
 ## Build Dependency
 ```shell
-sudo apt-get install libx11-dev libcairo2-dev
+sudo apt-get install libsdl2-dev libcairo2-dev
 ```
 
 ## Usage
@@ -14,4 +14,13 @@ make
 ./twin-fedit < nchars
 ```
 
-(press 'q' to next character)
+| Key binding | Function |
+|   ---       |   ---    |
+| `esc`       | Exit program |
+| `left mouse button` | Select a point as start point |
+| `right mouse button` | Select a point as end point |
+| `d` | Delete selected point|
+| `f`| Replace a line with a spline |
+|   `q`       | Next character |
+| `s` | Split a spline into two splines by start point and end point |
+| `u` | Undo the last operation |