I`ve read your code,you need to #define FXAA_GREEN_AS_LUMA 1,otherwise it will take the alpha of tex color as luminance,as your input alpha is always 1,nothing will be changed. And your parameters seemed to have some problems,try to read the annotation and check them.
If you want to understand the linear color space, try to google "gamma correction". Generally speaking, to the monitor, the relationship between luminance and color is not linear, but a Exponential curve. So in order to output the linear color, texture color was also been precorrected by an exponent value. So although you can see the correct color on the screen,the color you used before is nonlinear. This can cause problems in some algrithm,but in my work,the FXAA use nonlinear color space works well.