FXAA shader not used at best time

[slipher]

From the fxaa3_11_fp.glsl shader's integration instructions:

RGB should be LDR (low dynamic range).
Specifically do FXAA after tonemapping.

But we do it just before tonemapping, when the colors may be outside the 0-1 range.

Also sRGB may make this more important:

BEING LINEARLY CORRECT?
Applying FXAA to a framebuffer with linear RGB color will look worse.
This is very counter intuitive, but happens to be true in this case.
The reason is because dithering artifacts will be more visible
in a linear colorspace.

[illwieckz]

Fixed by:

• Fix and improve FXAA #1924