Move UTF-8 encoding to shared code

This will be used by the Wayland code too.

(cherry picked from commit cb22c54119)

src/init.c

@@ -111,6 +111,37 @@ static void terminate(void)
 //////                       GLFW internal API                      //////
 //////////////////////////////////////////////////////////////////////////
 
+// Encode a Unicode code point to a UTF-8 stream
+// Based on cutef8 by Jeff Bezanson (Public Domain)
+//
+size_t _glfwEncodeUTF8(char* s, unsigned int ch)
+{
+    size_t count = 0;
+
+    if (ch < 0x80)
+        s[count++] = (char) ch;
+    else if (ch < 0x800)
+    {
+        s[count++] = (ch >> 6) | 0xc0;
+        s[count++] = (ch & 0x3f) | 0x80;
+    }
+    else if (ch < 0x10000)
+    {
+        s[count++] = (ch >> 12) | 0xe0;
+        s[count++] = ((ch >> 6) & 0x3f) | 0x80;
+        s[count++] = (ch & 0x3f) | 0x80;
+    }
+    else if (ch < 0x110000)
+    {
+        s[count++] = (ch >> 18) | 0xf0;
+        s[count++] = ((ch >> 12) & 0x3f) | 0x80;
+        s[count++] = ((ch >> 6) & 0x3f) | 0x80;
+        s[count++] = (ch & 0x3f) | 0x80;
+    }
+
+    return count;
+}
+
 char* _glfw_strdup(const char* source)
 {
     const size_t length = strlen(source);