解决了。 iOS上的glReadPixels不支持RGBA到BGRA转换。
更改

Solved. RGBA to BGRA conversion is not supported by glReadPixels on iOS. Changing

glReadPixels(0, 0, _videoDimensions.width, _videoDimensions.height, GL_BGRA_EXT, GL_UNSIGNED_BYTE, CVPixelBufferGetBaseAddress(outPixelBuffer)); 

到

glReadPixels(0, 0, _videoDimensions.width, _videoDimensions.height, GL_RGBA, GL_UNSIGNED_BYTE, CVPixelBufferGetBaseAddress(outPixelBuffer));

解决问题。 glGetError是我的新朋友;）

Solves the problem. glGetError is my new friend ;)