add shader backend + support for multiple shader formats

scripts/shaders_d3d12.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 D3D12 DXIL shaders." >&2
+	exit 1
+fi
+
+mkdir -p shaders/d3d12
+shadercross shaders/sprite.vert.glsl -o shaders/d3d12/sprite.vert.dxil -s vertex -e main
+shadercross shaders/sprite.frag.glsl -o shaders/d3d12/sprite.frag.dxil -s fragment -e main