Add GrExternalTextureData and SkCrossContextImageData

GrExternalTextureData is an API for exporting the backend-specific
information about a texture in a type-safe way, and without pointing
into the GrTexture. The new detachBackendTexture API lets us release
ownership of a texture to the client.

SkCrossContextImageData is the public API that lets clients upload
textures on one thread/GrContext, then safely transfer ownership to
another thread and GrContext for rendering.

Only GL is implemented/supported right now. Vulkan support requires
that we add thread-safe memory pools, or otherwise transfer the
actual memory block containing the texture to the new context.

BUG=skia:

Change-Id: I784a3a74be69807df038c7d192eaed002c7e45ca
Reviewed-on: https://skia-review.googlesource.com/8529
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>

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