Added support for eglCreatePbufferFromClientBuffer(), using an
IOSurface on MacOS, or just a straight buffer pointer on other
platforms.

Added new unit tests (IOSurfaceClientBufferTest class), which
pass on both Windows and MacOS.

Change-Id: I79a6b420d85fb1f3ae505e0c0067bad2e27510d4
Reviewed-on: https://swiftshader-review.googlesource.com/17168Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>