fix ofxKinect isFrameNew logic

- The logic for isFrameNew was different than that in ofVideoPlayer where querying isFrameNew doesn't set it to false. Instead the update call sets it to true or false so you can call isFrameNew several times during a frame and it'll always return the same value

- isFrameNew was returning true no matter if the frame was new for depth or video which made the fps of about 60 and was uploading the textures more times than it was really needed. separating the flags in video and depth the measured fps is now 30 for both and textures are only uploaded when needed.

- There's now 2 isFrameNew methods, one for rgb and one for depth, the old one is kept for compatibility reasons and will return true if depth or rgb are new so it'll behave as before

- The member variable bUpdateTex wasn't used except inside draw and it wasn't really needed.