Shader Improvements

[lasers]

To allow users to undo their accident of applying one via keyboard (key:m and key:n)
To allow users to apply empty shader/noshader via keyboard (key:m and key:n) after trying some or all shaders.

• Add empty shader/noshader option to the shader list

To undo an intentional or accidental shader change, users will need to stop/pause their game, access menu/quick menu, find the shader option, make sure the number is at zero, and apply shader change before returning.

If that's not possible to add empty shader/noshader option because of coding, impossible to load empty shader from directory, or such... What about a mute button for shader?

Shader (current)

# Applies next and previous shader in directory.
# input_shader_next = m
# input_shader_prev = n

Shader (revised)

# Applies next and previous shader in directory. (NEW)
# input_shader_next = m
# input_shader_prev = n
# input_shader_toggle = comma

[hizzlekizzle]

The bilinear and nearest presets are essentially null shaders:
https://github.com/libretro/common-shaders/blob/master/bilinear.cgp
https://github.com/libretro/common-shaders/blob/master/nearest.cgp
They're in the base of the shader directories and you can cycle to/through them with m and n to remove all other shader effects.

[lasers]

I hadn't played with shaders yet... and I'm still playing games with no shaders. I may give them a try in the future. Right now, new users can accidentally press key:m or key:n and it will load ./shaders/retroarch.glslp by default. They then may want to undo the mistake by pressing key:m or key:n again only to find out that it does not work. I also didn't want a shader on right now.

Some can figure it out via Shader setting, but it's the longest (and the only) route to undo a shortcut. Others may not figure it out. They may ask online or restart game/RetroArch to get rid of an accidental shader.

It looks like the solution was to make ./shaders/no-shader.glslp with this line inside:
shaders = "0" and then you're able to use key:m and key:n to undo an easy mistake.

[lasers]

Okay. Out of the box, there is no problem with next/prev keys because there was nothing inside shader directory. As soon as you load some shader and hit Apply Shader Changes, it'll make ./shaders/retroarch.glslp with some random shader you pick. When you're done playing, save and leave RetroArch.

Start RetroArch again with a game of your choice, you accidentally hit next/prev keys instead of spacebar. You are stuck with the last shader you picked with a long way to undo. You will have to stop the game and go turn it off again... Again and again.

Adding that file will prevent the next/prev keys from "not working."

[lasers]

Users: Use the solution above if you want to fix this minor UI bug.

[dankcushions]

IMO it would still be better to include 'no shader' in the input_shader_next cycle. i guess 'bilinear'/'nearest neighbor' are effectively that, but it doesn't seem as elegant to me as simply turning off the shader.

it also would allow the user to create a shader on/off button via input_shader_next, and only one shader in their shaders directory. we have a use case for that here: https://retropie.org.uk/forum/topic/16720/crt-shaders-button-on-off (i don't know if my sneaky workaround will work, yet)

[dankcushions]

my sneaky workaround works :) see https://drive.google.com/file/d/1uKXfT0wzUSRx3V2-FSraPDpOk_R5TkeE/view

it's a .glslp file with one line: shaders = "0" :) a proper solution would be better, though.