Metal merge streams

Use Metal to merge CVPixelBuffers and resize CVPixelBuffer in hight performance.

Merge frames

• Time consuming 0.2 ~ 1.5 ms can make sure the living real-time.
• Inputs are tow CVPixelBuffers. CVPixelBuffer-A (from front camera) and CVPixelBuffer-B (from video 'rtpVideo.mp4'). Output is new CVPixelBuffers.
• Upload pixel buffer as MTLTexture and in-frame location to GPU
• MTLComputeCommandEncoder compute new texture

Resize frame

• Input is CVPixelBuffers. Output is new CVPixelBuffers.
• MPSImageBilinearScale to resize pixel buffer, support mode ScaleToFill,ScaleAspectFit,ScaleAspectFill

Tow method to create new pixel buufer.

With pixel buffer pool

• Time consuming 1 ~ 2 ms.
• Create output CVPixelBuffer from Pixel CVPixelBufferPool
• Create a new MTLTexture from output CVPixelBuffer
• Compute and wait for completion
• Return the result CVPixelBuffer which is IOSurface-backed

Without pixel buffer pool

• Time consuming 6 ~ 7 ms.
• Create a texture from MTLDevice
• Compute and wait for completion
• MTLBlitCommandEncoder copy computed texture from GPU to CPU as MTLBuffer
• Create output CVPixelBuffer from MTLBuffer, but the output CVPixelBuffer is not IOSurface-backed
• Deep copy output CVPixelBuffer and config the copied output is IOSurface-backed