Make joystick platform code init on demand

This makes joystick support initialize the first time a joystick
function is called, including those gamepad functions that are layered
on top of joystick functions.

Related to #1284.
Related to #1646.

src/internal.h

@@ -539,6 +539,7 @@ struct _GLFWlibrary
     _GLFWmonitor**      monitors;
     int                 monitorCount;
 
+    GLFWbool            joysticksInitialized;
     _GLFWjoystick       joysticks[GLFW_JOYSTICK_LAST + 1];
     _GLFWmapping*       mappings;
     int                 mappingCount;
@@ -632,6 +633,8 @@ void _glfwPlatformSetGammaRamp(_GLFWmonitor* monitor, const GLFWgammaramp* ramp)
 void _glfwPlatformSetClipboardString(const char* string);
 const char* _glfwPlatformGetClipboardString(void);
 
+GLFWbool _glfwPlatformInitJoysticks(void);
+void _glfwPlatformTerminateJoysticks(void);
 int _glfwPlatformPollJoystick(_GLFWjoystick* js, int mode);
 void _glfwPlatformUpdateGamepadGUID(char* guid);