Support 16/32 bit color depth

Mado currently supports 32-bit color depth for desktop and laptop
display devices. To enhance its compatibility across diverse Linux
framebuffer environments, I have made the following changes to support
both 16-bit and 32-bit color depths:

- Distinguish the color format and perform color format conversion.
- Examine the correctness of the framebuffer format based on the color
  format.

These updates improve Mado's compatibility, enabling it to be used on
devices with low memory overhead, such as the Raspberry Pi, in addition
to desktop display devices.

backend/fbdev.c

@@ -22,7 +22,9 @@
 #define FBDEV_DEFAULT "/dev/fb0"
 #define SCREEN(x) ((twin_context_t *) x)->screen
 #define PRIV(x) ((twin_fbdev_t *) ((twin_context_t *) x)->priv)
-
+#define RGB32_TO_ARGB8565(pixel)(((pixel & 0x00f80000) >> 8) | \
+                                 ((pixel & 0x0000fc00) >> 5) | \
+                                 ((pixel & 0x000000f8) >> 3) )
 typedef struct {
     twin_screen_t *screen;
 
@@ -59,7 +61,23 @@ static void _twin_fbdev_put_span(twin_coord_t left,
     off_t off = top * screen->width + left;
     uint32_t *dest =
         (uint32_t *) ((uintptr_t) tx->fb_base + (off * sizeof(*dest)));
-    memcpy(dest, pixels, width * sizeof(*dest));
+
+    /* If the framebuffer is in 16 bpp mode, convert each pixel from 32 bpp (ARGB) to 16 bpp (RGB565).
+    * - Extracts the red, green, and blue channels from the 32-bit color value.
+    * - Shifts and masks each channel to fit into the 16-bit RGB565 format:
+    *      - 5 bits for red
+    *      - 6 bits for green
+    *      - 5 bits for blue
+    * - Combines the shifted color values into a single 16-bit pixel.
+    * If the framebuffer is in 32 bpp mode, directly copy the pixels to the framebuffer.
+    */
+    if(tx->fb_var.bits_per_pixel == 16){
+        for (int i = 0; i < width; i++) {
+            dest[i] = RGB32_TO_ARGB8565(pixels[i]);
+        }
+    }else{
+        memcpy(dest, pixels, width * sizeof(*dest));
+    }
 }
 
 static void twin_fbdev_get_screen_size(twin_fbdev_t *tx,
@@ -88,6 +106,18 @@ static bool twin_fbdev_work(void *closure)
     return true;
 }
 
+static bool fb_is_rgb565(twin_fbdev_t* tx) {
+    return tx->fb_var.red.offset == 11 && tx->fb_var.red.length == 5 &&
+         tx->fb_var.green.offset == 5 && tx->fb_var.green.length == 6 &&
+         tx->fb_var.blue.offset == 0 && tx->fb_var.blue.length == 5;
+}
+
+static bool fb_is_argb32(twin_fbdev_t* tx) {
+    return tx->fb_var.red.offset == 16 && tx->fb_var.red.length == 8 &&
+         tx->fb_var.green.offset == 8 && tx->fb_var.green.length == 8 &&
+         tx->fb_var.blue.offset == 0 && tx->fb_var.blue.length == 8;
+}
+
 static bool twin_fbdev_apply_config(twin_fbdev_t *tx)
 {
     /* Read changable information of the framebuffer */
@@ -99,7 +129,6 @@ static bool twin_fbdev_apply_config(twin_fbdev_t *tx)
     /* Set the virtual screen size to be the same as the physical screen */
     tx->fb_var.xres_virtual = tx->fb_var.xres;
     tx->fb_var.yres_virtual = tx->fb_var.yres;
-    tx->fb_var.bits_per_pixel = 32;
     if (ioctl(tx->fb_fd, FBIOPUT_VSCREENINFO, &tx->fb_var) < 0) {
         log_error("Failed to set framebuffer mode");
         return false;
@@ -111,10 +140,17 @@ static bool twin_fbdev_apply_config(twin_fbdev_t *tx)
         return false;
     }
 
-    /* Check bits per pixel */
-    if (tx->fb_var.bits_per_pixel != 32) {
-        log_error("Failed to set framebuffer bpp to 32");
-        return false;
+    /* Examine the framebuffer format */
+    if (tx->fb_var.bits_per_pixel == 16) {
+        if(!fb_is_rgb565(tx)){
+            log_error("Invalid framebuffer format for 16 bpp");
+            return false;
+        }
+    }else{
+        if(!fb_is_argb32(tx)){
+            log_error("Invalid framebuffer format for 32 bpp");
+            return false;
+        }
     }
 
     /* Read unchangable information of the framebuffer */