Sample demo app

[megatran]

Hi! Thanks for this great library. I built a sample app here in case people want to quickly see it in action

Demo App: https://github.com/megatran/SIFTMetalApp

I made some minor changes to make it work on my phone

• The default SIFTMetal library uses the following parameters, which caused crashes on my device:

  private let maximumNumberOfExtrema = 4096
  private let maximumNumberOfKeypoints = 4096
  private let maximumNumberOfDescriptors = 2048

  I modified them to:

  private let maximumNumberOfExtrema = 16384
  private let maximumNumberOfKeypoints = 16384
  private let maximumNumberOfDescriptors = 16384

• I also modified the match rendering logic to support images of different sizes (see MatchRenderer.swiftg), instead of the original behavior in the package's SIFTRenderer.swift.