add shader backend + support for multiple shader formats

scripts/shaders_all.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+cd "$ROOT"
+
+./scripts/shaders_vulkan.sh
+./scripts/shaders_d3d12.sh
+./scripts/shaders_metal.sh

scripts/shaders_d3d12.sh

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+#!/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

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

scripts/shaders_vulkan.sh

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+#!/usr/bin/env bash
+set -euo pipefail
+mkdir -p shaders/vulkan
+glslangValidator -V shaders/sprite.vert.glsl -o shaders/vulkan/sprite.vert.spv
+glslangValidator -V shaders/sprite.frag.glsl -o shaders/vulkan/sprite.frag.spv
+