Added FXAA 3.11 GLSL Shader

Please review the shader because I am rather new to this.
(This is the first time I actually "developed" something in GLSL.)
It should work on older GPUs now, but I wasn't able to test it on one,
so I can't guarantee it will actually work.
Merge remote-tracking branch 'remotes/agreatfish/FXAA'
Rename EdgeFilter shader to FXAA2. Rename FXAA shader to FXAA3. Clean up FXAA3 GLSL code and port to HLSL.
Change __ to _ in the FXAA3 shader, as __ is reserved for GL internal use.