fix: color is too bright

Close #119

QtScrcpy/device/render/qyuvopenglwidget.cpp

@@ -61,15 +61,21 @@ static QString s_fragShader = R"(
     {
         vec3 yuv;
         vec3 rgb;
-        // SDL2-2.0.10\src\render\opengl\SDL_shaders_gl.c BT709_SHADER_CONSTANTS
+
+        // SDL2 BT709_SHADER_CONSTANTS
+        // https://github.com/spurious/SDL-mirror/blob/4ddd4c445aa059bb127e101b74a8c5b59257fbe2/src/render/opengl/SDL_shaders_gl.c#L102
         const vec3 Rcoeff = vec3(1.1644,  0.000,  1.7927);
         const vec3 Gcoeff = vec3(1.1644, -0.2132, -0.5329);
         const vec3 Bcoeff = vec3(1.1644,  2.1124,  0.000);
+
         // 根据指定的纹理textureY和坐标textureOut来采样
         yuv.x = texture2D(textureY, textureOut).r;
         yuv.y = texture2D(textureU, textureOut).r - 0.5;
         yuv.z = texture2D(textureV, textureOut).r - 0.5;
+
         // 采样完转为rgb
+        // 减少一些亮度
+        yuv.x = yuv.x - 0.0625;
         rgb.r = dot(yuv, Rcoeff);
         rgb.g = dot(yuv, Gcoeff);
         rgb.b = dot(yuv, Bcoeff);