Implement user-specified pixel shaders, redux

[zadjii-msft]

Co-authored-by: mrange marten_range@hotmail.com

I loved the pixel shaders in #7058, but that PR needed a bit of polish
to be ready for ingestion. This PR is almost exactly that PR, with
some small changes.

• It adds a new pre-profile setting "experimental.pixelShaderPath",
  which lets the user set a pixel shader to use with the Terminal.
  • CHANGED FROM Implemented support for user provided pixel shaders #7058: It does not add any built-in shaders.
  • CHANGED FROM Implemented support for user provided pixel shaders #7058: it will override
    experimental.retroTerminalEffect
• It adds a bunch of sample shaders in samples/shaders. Included:
  • A NOP shader as a base to build from.
  • An "invert" shader that inverts the colors, as a simple example
  • An "grayscale" shader that converts all colors to grayscale, as a
    simple example
  • An "raster bars" shader that draws some colored bars on the screen
    with a drop shadow, as a more involved example
  • The original retro terminal effects, as a more involved example
  • It also includes a broken shader, as an example of what heppens
    when the shader fails to compile
  • CHANGED FROM Implemented support for user provided pixel shaders #7058: It does not add the "retroII" shader we were
    all worried about.
• When a shader fails to be found or fails to compile, we'll display an
  error dialog to the user with a relevant error message.
  • CHANGED FROM Implemented support for user provided pixel shaders #7058: Originally, Implemented support for user provided pixel shaders #7058 would display "error bars"
    on the screen. I've removed that, and had the Terminal disable the
    shader entirely then.
• Renames the toggleRetroEffect action to toggleShaderEffect.
  (toggleRetroEffect is now an alias to toggleShaderEffect). This
  action will turn the shader OR the retro effects on/off.

toggleShaderEffect works the way you'd expect it to, but the mental
math on how is a little weird. The logic is basically:

useShader = shaderEffectsEnabled ? 
                (pixelShaderProvided ? 
                    pixelShader : 
                    (retroEffectEnabled ? 
                        retroEffect : null
                    )
                ) : 
                null

and toggleShaderEffect toggles shaderEffectsEnabled.

• If you've got both a shader and retro enabled, toggleShaderEffect
  will toggle between the shader on/off.
• If you've got a shader and retro disabled, toggleShaderEffect will
  toggle between the shader on/off.

References #6191
References #7058

Closes #7013

Closes #3930 "Add setting to retro terminal shader to control blur
radius, color"
Closes #3929 "Add setting to retro terminal shader to enable drawing
scanlines"
- At this point, just roll your own version of the shader.

[WSLUser]

So https://stackoverflow.com/questions/15095909/from-rgb-to-hsv-in-opengl-glsl had an update for the license after requesting it. It's under https://en.wikipedia.org/wiki/WTFPL. Can this be looked into legally if it can be ingested like it originally was going to be? It would be extremely useful if it could be. If it's able to be, that can be a separate PR. We've already waited long enough for this.

[DHowett]

@WSLUser Thanks for that! The rgb-hsv routines weren't my only concern, but this helps.

[DHowett]

i hope the whitespace change here doesn't cause a re-loc

[DHowett]

Let's do this.

[ghost]

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