GPUImageFramebufferCache: remove defunct framebufferCacheQueue
GPUImageFramebufferCache: simplify caching
Framebuffer pool for GPUImageFramebufferCache

Including idle frame buffer purging.
Experiment with glFlush() instead of glFinish() in GPUImageCoreVideoOutput

As recommended by the OpenGL ES Analyzer. The documentation I read suggests that you're unlikely to need this... it feels like maybe we need it so the video buffer is definitely populated by the time we need it but we'll see.