add shader backend + support for multiple shader formats

scripts/shaders_metal.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if ! command -v shadercross >/dev/null 2>&1; then
+	echo "shadercross not found (SDL_shadercross). Install it to build Metal MSL shaders." >&2
+	exit 1
+fi
+
+mkdir -p shaders/metal
+shadercross shaders/sprite.vert.glsl -o shaders/metal/sprite.vert.msl -s vertex -e main
+shadercross shaders/sprite.frag.glsl -o shaders/metal/sprite.frag.msl -s fragment -e main