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.

Re-land of https://skia-review.googlesource.com/c/8529/

BUG=skia:

Change-Id: I48ebd57d1ea0cfd3a1db10c475f2903afb821966
Reviewed-on: https://skia-review.googlesource.com/8960
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/src/gpu/vk/GrVkCaps.cpp b/src/gpu/vk/GrVkCaps.cpp
index 94c1a50..fd454ad 100644
--- a/src/gpu/vk/GrVkCaps.cpp
+++ b/src/gpu/vk/GrVkCaps.cpp
@@ -37,6 +37,7 @@
 
     fUseDrawInsteadOfClear = false;
     fFenceSyncSupport = true;   // always available in Vulkan
+    fCrossContextTextureSupport = false; // TODO: Add thread-safe memory pools so we can enable this
 
     fMapBufferFlags = kNone_MapFlags; //TODO: figure this out
     fBufferMapThreshold = SK_MaxS32;  //TODO: figure this out