This commit introduces IOSGLMemory which is a GLMemory that falls back to
GstAppleCoreVideoMemory for CPU access. This is a temporary solution until
IOSurface gets exposed as a public framework on iOS and so we can use
IOSurfaceMemory on both MacOS and iOS.
https://bugzilla.gnome.org/show_bug.cgi?id=769210
Rather than assuming something. e.g. zerocopy on iOS with GLES3 requires
the use of Luminance/Luminance Alpha formats and does not work with
Red/RG textures.
Switch to using IOSurface instead of CVOpenGLTextureCache on OSX. The latter can't be
used anymore to do YUV => RGB with opengl3 on El Capitan as GL_YCBCR_422_APPLE
has been removed from the opengl3 driver. Also switch to NV12 from UYVY, which
was the only YUV format supported by CVOpenGLTextureCache.
