Test player does not render any frame on OSX

[ferjm]

All I see is an empty application window when running the test player on OSX.

The output says: Rendered 0 frames in 5.934178627001529 seconds; 0 fps.

I believe the regression was introduce with #131 as reverting to a previous commit makes everything work as expected.

[cpearce]

This is caused by to_ffi_planar_ycbycr_images() failing to retrieve a buffer for video frames because I forgot to include the "return" keyword in my definition of RustBufferAllocator::Allocate(). Yay unsafe code.