Fix rendering to a destroyed gralloc buffer.

Wait for draw calls that use the Android native image to finish before
dereferencing the buffer and potentially destroying it.

Bug 20885669

Change-Id: I1c8f56e5065ad4c77d7b950feec4505c2e109a47
Reviewed-on: https://swiftshader-review.googlesource.com/3074
Reviewed-by: Greg Hartman <ghartman@google.com>
Tested-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>