don't bind resources unnecessarily

src/host_shaders/metal_blit.metal

@@ -0,0 +1,29 @@
+#include <metal_stdlib>
+using namespace metal;
+
+#define GET_HELPER_TEXTURE_BINDING(binding) (30 - binding)
+#define GET_HELPER_SAMPLER_STATE_BINDING(binding) (15 - binding)
+
+struct BasicVertexOut {
+	float4 position [[position]];
+	float2 uv;
+};
+
+struct NDCViewport {
+    float2 offset;
+    float2 scale;
+};
+
+vertex BasicVertexOut vertexBlit(uint vid [[vertex_id]], constant NDCViewport& viewport [[buffer(0)]]) {
+	BasicVertexOut out;
+	out.uv = float2((vid << 1) & 2, vid & 2);
+	out.position = float4(out.uv * 2.0 - 1.0, 0.0, 1.0);
+	out.position.y = -out.position.y;
+	out.uv = out.uv * viewport.scale + viewport.offset;
+
+	return out;
+}
+
+fragment float4 fragmentBlit(BasicVertexOut in [[stage_in]], texture2d<float> tex [[texture(GET_HELPER_TEXTURE_BINDING(0))]], sampler samplr [[sampler(GET_HELPER_SAMPLER_STATE_BINDING(0))]]) {
+	return tex.sample(samplr, in.uv);
+}