update SDL3 from 3.2.20 to 3.4.2

libs/SDL3/test/testgpurender_effects_grayscale.frag.hlsl

@@ -0,0 +1,19 @@
+Texture2D u_texture : register(t0, space2);
+SamplerState u_sampler : register(s0, space2);
+
+struct PSInput {
+    float4 v_color : COLOR0;
+    float2 v_uv : TEXCOORD0;
+};
+
+struct PSOutput {
+    float4 o_color : SV_Target;
+};
+
+PSOutput main(PSInput input) {
+    PSOutput output;
+    float4 color = u_texture.Sample(u_sampler, input.v_uv) * input.v_color;
+    float gray = color.r * 0.2126 + color.g * 0.7152 + color.r * 0.722;
+    output.o_color = float4(gray, gray, gray, color.a);
+    return output;
+}