Rewrite host GPU abstraction

- Don't have to repeat the same thing for 4 renderers.
 - Add native Metal renderer.

src/scmversion/gen_scmversion.sh

@@ -4,7 +4,7 @@ VERSION_FILE="scmversion.cpp"
 
 CURDIR=$(pwd)
 if [ "$(uname -s)" = "Darwin" ]; then
-  cd "$(dirname $(python -c 'import os,sys;print(os.path.realpath(sys.argv[1]))' "$0"))"
+  cd "$(dirname $(python3 -c 'import os,sys;print(os.path.realpath(sys.argv[1]))' "$0"))"
 else
   cd $(dirname $(readlink -f $0))
 fi