Port font editor to SDL2

Port font editor from X11 to SDL2, improving portability,
cross-platform support, and maintainability.

Window creation now uses SDL_CreateWindow, replacing X11 XCreateWindow.
Event handling is migrated to SDL2 SDL_PollEvent, streamlining
the capture of keyboard and mouse inputs.

Rendering is updated to leverage SDL_Renderer and SDL_Surface,
replacing X11 rendering functions.

Key event behaviors have been refined for better usability.
Pressing SDLK_ESCAPE or clicking the window close button (SDL_QUIT)
now exits the application.

To address a formatting issue with clang-format,
the function delete() has been renamed to delete_first_cmd(),
avoiding conflicts with the C++ keyword delete.

File reading has also been simplified using fopen for improved clarity.

The README has been expanded with a background on twin-fedit,
key bindings, a quick start guide,
and a demo GIF to help users get started.

Close #7

Author: jouae

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

@@ -1,17 +1,93 @@
 # twin-fedit
 `twin-fedit` is a tool allowing users to edit specific scalable fonts
-which are expected to fit the requirements of embedded systems with larger
-screens.
+which are expected to fit the requirements of embedded systems with larger screens.
+
+<p align="center">
+  <img src="./assets/demo.gif" />
+</p>
 
 ## Build Dependency
 ```shell
-sudo apt-get install libx11-dev libcairo2-dev
+sudo apt-get install libsdl2-dev libcairo2-dev
 ```
 
 ## Usage
 ```shell
 make
-./twin-fedit < nchars
+./twin-fedit nchars
+```
+
+## Background
+The glyphs in `twin-fedit` is originated from [Hershey vector fonts](https://en.wikipedia.org/wiki/Hershey_fonts), which were created by Dr. A. V. Hershey while working at the U. S. National Bureau of Standards.
+
+The Hershey vector fonts set of `twin-fedit` is [`nchars`](nchars), for example, the interpolation points and operations used to draw the glyph `1` are as follows
+```
+/* 0x31 '1'  offset 666 */
+    0, 10, 42, 0, 2, 3,
+    0, 10, /* snap_x */
+    -21, -15, 0, /* snap_y */
+    'm', 0, -34,
+    'c', 4, -35, 8, -38, 10, -42,
+    'l', 10, 0,
+    'e',
 ```
 
-(press 'q' to next character)
+The first line to 4-th line are used to draw the glyph.
+
+The first line of is the header that contains the information of the character `1`, `0x31` is the ASCII code of `1` and offset `666` is the 
+
+The character `m` is an abbreviation for `move to`, and the values following `m` represent the x and y positions to move to in the drawing window's coordinate system, respectively.
+
+The character `c` is an abbreviation for `curve to`, and the values following `c` represent three x-y coordinate points used to draw a cubic Bézier curve, in the order of the first control point, the second control point, and the endpoint.
+
+The character `l` is an abbreviation for `line to`, and the values following `l` represent the x and y positions to move to, relative to the position from the previous operation, in the drawing window's coordinate system, respectively.
+
+The character `e` is an abbreviation for `end`.
+
+According to the steps outlined above for drawing glyph `1`, it can be split into the following steps:
+
+1. `'m' 0,-34`: Move to `0,-34` and plot a point at `0,-34`.  
+2. `'c' 4, -35, 8, -38, 10, -42`: Starting from `0,-34` and ending at `10,-42`, draw a curve using Bézier curve with the first control point at `4,-35` and the second control point at `8,-38`.  
+3. `'l' 10,0`: Starting from `10,-42` and ending at `10,0`, draw a straight line.  
+4. `'e'`: End the drawing of glyph `1`.
+
+Each point seen in `twin-fedit` corresponds to an operation. By selecting a point in `twin-fedit`, you can modify the coordinates to edit any glyph.
+
+## Quick Guide
+For each glyph, there are the following shortcut keys used for editing the glyph.
+
+| Key  | Functionality |
+|   ---       |   ---    |
+|  ESC  | Exit program |
+| left mouse button | Select a point as the first operation for that glyph |
+| right mouse button | Select a point as the last operation for that glyph |
+| d | Delete selected point|
+| f | Replace a line with a spline |
+| q | Switch to next character |
+| s | Split a spline into two splines by start point and end point |
+| u | Undo the last operation |
+
+
+To move a point
+1. Select a point by left mouse button,
+2. Use arrow keys to move the selected point.
+
+To move a control point
+1. Select a point with two control points by left mouse button,
+2. Use arrow keys to move the first control point,
+3. Keep pressing shift key and use arrow keys to move the second control point.
+
+To split a spline or line
+1. Select two point by left and right mouse button,
+2. Use s key to split the line or spline into two segments.
+
+To replace the line or spline by another spline
+1. Select two point by left and right mouse button as start and end of the another spline,
+2. Use f key to replace the line or spline with another spline.
+
+To delete a point
+1. Select a point by left mouse button,
+2. Use d key to delete the selected point.
+
+To undo any operations above
+1. Use u key.