Previously, Streams had two modes (native and texid), this adds a third
mode called "acquired", which allows for copy-free synchronized external
textures in OpenGL and paves the way for Vulkan.
The native mode is now deprecated but texid mode needs to stay around
until all clients can be upgraded.
In an early prototype, this functionality was added directly into
Texture but required quite a bit of additional state tracking, so the
Stream API seemed like a better fit.
This API is probably not necessary for Metal due to Metal's shared
ownership semantics.
This has been tested with a new Android sample that will be added in a
subsequent PR.
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