Move UTF-8 encoding to shared code

This will be used by the Wayland code too.

src/internal.h

@@ -995,6 +995,8 @@ GLFWbool _glfwInitVulkan(int mode);
 void _glfwTerminateVulkan(void);
 const char* _glfwGetVulkanResultString(VkResult result);
 
+size_t _glfwEncodeUTF8(char* s, unsigned int ch);
+
 char* _glfw_strdup(const char* source);
 float _glfw_fminf(float a, float b);
 float _glfw_fmaxf(float a, float b);