オブジェクトを不透明にするソーティングモード
Opaque objects are sorted by various criteria (sorting layers, shader queues,
materials, distance, lightmaps etc.) to maximize both the CPU efficiency (reduce number
of state changes and improve draw call batching), and to maximize GPU efficiency
(many GPUs prefer rough front-to-back rendering order for faster rejection of invisible
surfaces).
By default, opaque objects are grouped in rough front-to-back buckets, on the GPUs
where doing that is beneficial. There are GPUs where doing this distance based
sorting is not really helpful (most notably, PowerVR/Apple GPUs), and so on these GPUs
the distance based sorting is not done by default.
The Camera.opaqueSortMode property lets you override this default behavior.
For example, you might want to never do distance-based sorting for opaque objects,
if you know you need much more CPU performance than GPU performance.
参考： OpaqueSortMode enum、transparencySortMode