EXT_external_buffer spec -
    This extension allows the data store for an immutable buffer to be
    sourced from an external EGLClientBuffer, allowing sharing of EGL
    client buffers across APIs, across processes, and across different
    processing cores such as the GPU, CPU, and DSP.

The intent is for a single backing memory to be reused across various
processes and processors. Ensure that a glBuffer backed by external
memory does not orphan the memory when glBuffer APIs like
glBufferSubData or glMapBufferRangeEXT modify the glBuffer.

Bug: angleproject:4380
Bug: angleproject:5073
Tests: ExternalBufferTestES31.*DoesNotCauseOrphaning*Vulkan
Change-Id: I4e88f80d93ee1ba1208378121412926351d10af8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2776192
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>