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.

Author: huaxinliao

backend/fbdev.c

@@ -88,6 +88,27 @@ static bool twin_fbdev_work(void *closure)
     return true;
 }
 
+static inline bool twin_fbdev_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 inline bool twin_fbdev_is_rgb888(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 inline bool twin_fbdev_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 */
@@ -111,10 +132,29 @@ 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 */
+    switch (tx->fb_var.bits_per_pixel) {
+    case 16: /* RGB565 */
+        if (!twin_fbdev_is_rgb565(tx)) {
+            log_error("Invalid framebuffer format for 16 bpp");
+            return false;
+        }
+        break;
+    case 24: /* RGB888 */
+        if (!twin_fbdev_is_rgb888(tx)) {
+            log_error("Invalid framebuffer format for 24 bpp");
+            return false;
+        }
+        break;
+    case 32: /* ARGB32 */
+        if (!twin_fbdev_is_argb32(tx)) {
+            log_error("Invalid framebuffer format for 32 bpp");
+            return false;
+        }
+        break;
+    default:
+        log_error("Unsupported bits per pixel: %d", tx->fb_var.bits_per_pixel);
+        break;
     }
 
     /* Read unchangable information of the framebuffer */