Clamp chroma texcoords to avoid sampling alignment padding

This resolves #885 without the massive perf hit on lower end Intel GPUs.

app/shaders/d3d11_video_pixel_start.hlsli

@@ -6,4 +6,9 @@ struct ShaderInput
 {
     float4 pos : SV_POSITION;
     float2 tex : TEXCOORD0;
-};
+};
+
+cbuffer ChromaLimitBuf : register(b0)
+{
+    min16float3 chromaTexMax;
+};