updated chromakey blend filter

the chromakey blend filter taken from the iOS version wasn't fully ported correctly. it was using the wrong fragment shader. also - added filter tool updates to use the slider with the threshold value for simple sliding, updated the base threshold to 0.4 (on 0.3 the green screen testing i did looked like it was broken and slider wasnt working so i was pounding my head for days haha)

additionally in GPUImage you should consider putting a getter for the surfaceview since i couldnt set other properties like the pixel format etc, so i just put the base ones for RGBA8888 in the init call.