Add sgamepad_is_connected

[meshula]

• adds a return value to sgamepad_get_gamepad_state to indicate whether the read values come from a connected joystick. (If return == index, success)
• adds a connected field to sgamepad_gamepad_state
• adds bool sgamepad_is_connected(unsigned int index)
• uses float_min and float_max instead of fmin fmax to solve MSVC issues that cause upcasting to double
• eliminates __cplusplus check on ARC features to enable use from with with ObjC++ in addition to ObjC
• logic not tested on Android because I'm not set up for Android development