MetalSurfaceEXT and MacOSSurfaceMVK were inheriting incorrectly from
the base class MetalSurface, and were improperly initialized.
Additionally, none of the drawables were ever released, meaning there
were no re-used drawables. Add an @autoreleasepool around the drawable
retrival path, as per the spec: https://developer.apple.com/documentation/quartzcore/cametallayer?language=objc#3172024

Bug: chromium:1015454
Change-Id: Idd2f1842977a6f5dccecf31d152d454a44e9261c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/37948Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Jonah Ryan-Davis <jonahr@google.com>