Don't expose VK_EXT_external_memory_host on macOS
Don't expose VK_EXT_external_memory_host on macOS

macOS has strong assumptions about GPU availability, and thus doesn't
have a reliable API for sharing memory between the GPU and SwiftShader
which can comply with SwiftShader's alignment and padding needs.
Specifically, we must not try obtaining a pointer from an IOSurface and
import it into SwiftShader.

Bug: angleproject:7538
Bug: b/178039602
Change-Id: I3ecc4dff31a9e025f50c4afcf7d82fa4384b2cd1
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/67168
Kokoro-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Alexis Hétu <1c7141772178a7e80858c3db5052b328076f08f4@google.com>
Tested-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>

google/swiftshader,google/swiftshader,google/swiftshader